In a putative class of twenty-two large and sophisticated corporations, most of which have multi-million dollar claims, alleging a global conspiracy involving Cephalon and all four generic defendants under 15 U.S.C. section 1, four separate conspiracies between Cephalon and each generic defendant under the same statute, and a monopolization claim against Cephalon under 15 U.S.C. section 2, the District Court's class certification order is vacated where plaintiffs have not met their burden of showing that the numerosity requirement of Rule 23(a)(1) has been satisfied, and remanded for further consideration of whether joinder of all class members is impracticable.