GB24, a mouse monoclonal antibody, recognizes a trophoblast–leukocyte cross-reactive antigen (TLX), which is likely identical to the membrane cofactor protein (MCP), a complement regulatory protein. GB24 reacts also with a human acrosomal sperm antigen (Fénichel et al.: J Reprod Fertil 87: 699–706, 1989). By immunofluorescence or immunoperoxidase, testicular, epididymal, and ejaculated spermatozoa were found to be positive after fixation by acetone. Motile, suspended spermatozoa became positive only through conditions known to induce acrosome reaction (A23187, follicular fluid, contact with oocytes). Ultrastructural studies with immunogold staining localized this protein on the inner acrosome membrane and in the acrosomal content. By SDS-polyacrylamide gel electrophoresis, GB24 immunoprecipitated a unique protein of 48 kDa from capacitated and A23187–induced spermatozoa under reducing conditions. No cross-reactivity was found with mouse, boar, or ram spermatozoa. Localization of this human sperm antigen recognized by GB24 and its similarity with the TLX-MCP family antigens would suggest a possible role of this molecule during fertilization in sperm–egg binding or immune protection.