The plaintiffs, in their capacities as trustees of the Plumbers’ Union Local No. 12 Education, Apprenticeship, Industry Improvement, Pension, Welfare, Annuity, and Christmas and Vacation Funds (the funds), commenced this action in the Superior Court, alleging that the *535defendant, a plumbing contractor, had breached his contract with Local No. 12 (the Union) by failing to make monthly contributions to the funds at specified rates. The defendant denied the existence of an agreement requiring such contributions, and he filed a counterclaim for the monies collected by the plaintiffs from him as contributions to the funds on the basis that these contributions had been made in violation of 29 U.S.C. § 186 (1976). The case was referred to a master, who made findings of fact which were generally favorable to the plaintiffs. The judge modified the master’s report in certain respects not here material, and, as modified, he confirmed it and entered a judgment for the plaintiffs. We affirm the judgment.

We summarize the relevant facts as they appear from the master’s findings of subsidiary facts, which we accept as they are not mutually inconsistent, contradictory, plainly wrong or vitiated in view of the controlling law, see Covich v. Chambers, 8 Mass. App. Ct. 740, 743 (1979), and cases cited, and as they appear from the documents in evidence. In January, 1959, the defendant and the Union executed a written contract whereby the defendant agreed to comply with all of the requirements of the collective bargaining agreement between the Union and the Master Plumbers Association of Boston (the Association), an employer’s association to which the defendant has never belonged.2 The 1957 collective bargaining agreement expired in September, 1959, and it has been succeeded by a new collective bargaining agreement every two or three years. The defendant never signed the 1957 or succeeding collective bargaining agreements.

These collective bargaining agreements all contained provisions requiring employer contributions, in specified amounts, to various trust funds established over the years in accord with § 302(c)(5) of the Labor-Management Relations Act of 1947, 61 Stat. 157 (1947), 29 U.S.C. § 186(c)(5) (1976). The *536amounts of the contributions were determined on the basis of written reports which each employer was required to file and which showed the number of hours worked each month by Union plumbers. From 1959 through November, 1976, the defendant employed Union plumbers, and he made contributions to the Union funds in the amounts determined on the basis of the reports he filed.

On September 1,1976, a new collective bargaining agreement took effect between the Union and the Association. Under this agreement, an employer was to pay the plaintiffs $2.78 per hour for each Union plumber employed by him.* *3 Beginning in December, 1976, the defendant became delinquent in these payments to the funds, although he faithfully filed with the Union the report forms required by the collective bargaining agreement.4 In May, September, and October, 1977, the plumbers ceased work on the defendant’s jobs because of his payment delinquencies. On each occasion, however, the plumbers returned to work after the defendant, who acknowledged his indebtedness to the funds, was able to negotiate terms for payment of the arrearages. Union plumbers again ceased work on the defendant’s jobs in November, 1977, and no Union plumbers have worked for him since that time. Federal statute 29 U.S.C. § 186(a) (1976), prohibits any payment by an employer to an employee representative. Section 186(c) of 29 U.S.C. sets out certain clearly defined exceptions to that general prohibition. The exception provided by § 186(c)(5) permits employer payments to trust funds established by the employee representative “for the sole and exclusive benefit of the employees of such employer, and their families and dependents (or of such employees, families, and dependents jointly with the *537employees of other employers making similar payments, and their families and dependents)” so long as, among other requirements not here pertinent, “the detailed basis on which payments are to be made is specified in a written agreement with the employer.” 29 U.S.C. § 186(c)(5)(B) (1976).

The defendant argues that he has no written agreement with the Union or the plaintiffs providing a detailed basis for the payment to the funds and that such payments are, therefore, illegal under § 186(a), and uncollectible in this civil action. In support of his counterclaim, the defendant maintains that the prior payments which he made to the funds were illegal and must be returned to him.5

It is undisputed that the 1976 collective bargaining agreement between the Union and the Association sets forth in adequate detail the basis for employer payments to the funds. The master’s findings of subsidiary facts and the documentary evidence incorporated by reference in his report establish that the defendant and the Union agreed to adopt this collective bargaining agreement as their own. See Roadway Exp., Inc. v. Teamsters Local 249, 330 F.2d 859, 863 (3d Cir. 1964); Line Drivers Local No. 961 v. W. J. Digby, Inc., 218 F. Supp. 519, 522-523 (D. Colo. 1963), affd, 341 F.2d 1016 (10th Cir. 1965). Compare Central Appalachian Coal Co. v. United Mine Wkrs., 376 F. Supp. 914, 921 (S.D. W.Va. 1974). This evidence shows that the parties defined their relationship by reference to the 1959 and succeeding collective bargaining agreements.6 Contrast *538Moglia v. Geoghegan, 403 F.2d 110, 118 (2d Cir. 1968), cert. denied, 394 U.S. 919 (1969). See Bricklayers Local No. 15 v. Stuart Plastering Co., 512 F.2d 1017, 1029 (5th Cir. 1975). The Union furnished the defendant with plumbers, and the defendant complied with the payment and reporting requirements of the succeeding agreements for nearly eighteen years. Contrast Wagor v. Cal Kovens Constr. Co., 382 F.2d 813, 815 (5th Cir. 1967), cert. denied, 390 U.S. 952 (1968); Local No. 529, United Bhd. of Carpenters v. Bracy Dev. Co., 321 F. Supp. 869, 874-875 (W.D. Ark. 1971). This evidence supports the masters findings to the effect that the Union and the defendant agreed to be bound by the collective bargaining agreement in effect between the Union and the Association when the present dispute arose. On this record the plaintiffs have shown “the underlying elements of mutual consent and obligation which must be manifested in some form,” id. at 875, to establish the adoption of a collective bargaining agreement. Contrast Moglia v. Geoghegan, 403 F.2d at 118.

The defendant next contends that even if he did adopt the collective bargaining agreement between the Union and the Association, his contribution to the fund is precluded by the language of the trust agreements by which the funds were established. In particular, he argues that he is not an “employer,” as that term is defined in the trust instruments, from whom the instruments allow payments to be received. We are unpersuaded by this argument. The trust agreements were received in evidence by the master, and he incorporated them by reference in his report. One such document to which the defendant points is entitled “Plumbers Union, Local No. 12 Annuity Plan,” and it defines “Em*539ployer” as one “who has executed [a] collective bargaining agreement[ ] with the Union, or other written agreement with the Union or the trustees, which agreement[ ] provide[s] for payments to the fund. It is unnecessary to decide whether the defendant has “executed” such an agreement in the usual sense of that term because this document does not appear to have been executed prior to 1978, and is, therefore, not material to the legality of the payments due in 1976 and 1977. See Stuart Plastering Co., 512 F.2d at 1026. It appears that the legality of the payments for those years would be determined by reference to an “Agreement and Declaration of Trust” which the Annuity Plans refer to as already in existence. That agreement and declaration of trust has not been included in the record, and the defendant has failed to show that acceptance of his payments was illegal.

The foregoing discussion also disposes of the defendant’s contentions in support of his counterclaim.

Judgment affirmed.