A substantially purified intra-acrosomal primate sperm antigen useful in a contraceptive vaccine is disclosed herein. The antigen remains associated with primate sperm after the acrosome reaction. In particular, it remains associated with the inner and outer acrosomal membranes. Modified antigens and fragments thereof prepared by protein modification techniques are also disclosed as well as methods for purifying and using the antigens. Also disclosed are monoclonal and polyclonal antibodies to the antigen and methods of making and using such antibodies. Methods of use include purification of the antigen or use in various diagnostic techniques. Also disclosed are cDNA, expression vectors, and transformed microorganisms that produce the antigen.