Human CD52 antigen is a highly glycosylated molecule with an unusually small core peptide exclusively expressed on lymphocytes and mature sperm. In the male reproductive tract, it is secreted mainly from the epididymis and inserted into the sperm membrane via the glycosyl-phosphatidyl inositol (GPI) anchor during the passage of the spermatozoa through the epididymis. It has recently been found that the male reproductive tract CD52 (mrt-CD52) is a target antigen of human monoclonal antibody (Mab H6-3C4) obtained from an antisperm antibody-mediated infertile woman. The Mab H6-3C4 shows strong sperm-immobilizing activity with complement and specifically recognizes the N-linked carbohydrate epitope of sperm CD52 but not lymphocyte CD52. Lectin binding assays have revealed the presence of both O-linked as well as N-linked carbohydrate moieties in human mrt-CD52. Mouse monoclonal antibody (1 G12) reacting to human mrt-CD52 strongly inhibits penetration of human spermatozoa to the zona denuded hamster oocyte. Mouse CD52 is similar to human CD52 in biological and immunological characteristics. Male and female mice immunized with naturally-occurring mouse mrt-CD52 molecules produce antibodies against the cognate antigen yielding antisera with complement-dependent mouse sperm immobilizing activities.