Respondent moves for reconsideration of our ex parte order allowing an interlocutory appeal, pursuant to the provisions of 28 U.S.C. § 1292(b), from a discovery order of the district court in an Internal Revenue subpoena matter. His principal contention is that there is not a “controlling question of law,” as statutorily required. We have assumed, without deciding, in similar circumstances that there would be such a question. Goldfine v. Pastore, 1 Cir., 1958, 261 F.2d 519, 521. On further consideration we believe there is not. Pre-trial disclosure may indeed involve an ultimate question of law in the case, Commonwealth Edison Co. v. Allis-Chalmers Mfg. Co., N.D.Ill, 1963, 225 F.Supp. 332, aff’d 335 F.2d 203, but it may not. Here the only question is the admissibility of certain evidence on a legally relevant, Lash v. Nighosian, 1 Cir., 1959, 273 F.2d 185, cert. denied 362 U.S. 904, 80 S.Ct. 610, 4 L.Ed.2d 554, issue. This is not a controlling question of law. United States v. Woodbury, 9 Cir., 1959, 263 F.2d 784. We prefer the dissenting to the majority opinion in Groover, Christie & Merritt v. LoBianco, 1964, 119 U.S.App.D.C. 50, 336 F.2d 969.

The order allowing an interlocutory appeal is vacated. If the matter be thought sufficiently serious, a petition for mandamus may be filed. Cf. Lear Siegler, Inc. v. Adkins, 9 Cir., 1964, 330 F.2d 595. We need hardly say that the burden here is much heavier, and a very substantial showing of prejudice must be made before we would even consider such a petition. Cf. Switzerland Cheese Ass’n, Inc. v. Horne’s Market, Inc., 1 Cir., 1965, 351 F.2d 552, aff'd 385 U.S. 23, 87 S.Ct. 193, 17 L.Ed.2d 23.