To dissect the process of mammalian sperm interaction with the egg at a molecular level, we have generated monoclonal antibodies (mABs) to mature mouse sperm using syngeneic mouse testis as the immunogen. In this paper, we report upon three members of a mAB family, all of which displayed identical immunofluorescence patterns on cauda epididymal mouse sperm. Each of these mABs, termed M42, M5, and M41, localized to a restricted region of plasma membrane overlying the acrosome. When tested for an effect on the fertilization process in vitro, two of the mAbs, M42 and M5, demonstrated significant inhibition. The inhibitory capacity was dependent upon the presence of the zona pellucida; neither M42 nor M5 was capable of blocking fertilization when zona pellucida-free mouse eggs were used. Identification of the antigens recognized by this group of mAbs was achieved by immunologic detection of sodium dodecyl sulfate-extracted sperm components separated via electrophoresis on 12% sodium dodecyl sulfate-polyacrylamide gels followed by transfer to nitrocellulose, M42, which blocked fertilization, recognized a high molecular weight cluster of bands with M(r) of approximately 220,000 to 240,000. M5, which also prevented fertilization, specifically recognized a sperm component with subunit molecular weight of approximately 54,000. M41, which did not interfere with fertilization, did not interact with any high molecular weight components, but recognized components with M(r) of approximately 60,000, 35,000 and 21,000. Taken together with the work presented in a companion paper ( Sailing, Irons, and Waibel, this issue), we have demonstrated that it is possible to describe particular cellular regions of mammalian sperm with respect not only to location and function, but also to the molecules that are candidates for a role in that function.