Mouse seminal plasma (SP), a mixture of aqueous extracts of prostate, seminal vesicle, and epididymis, exerts potent immunosuppressive effects in vivo. The SP mixture completely suppressed both primary and secondary humoral immune responses in mice to low immunizing doses of antigen (bovine serum albumin or washed epididymal sperm), and also significantly suppressed the antibody response to high doses of immunizing antigen. The humoral immune response to epididymal sperm was also suppressed when sperm were incubated in SP and then washed before immunization and when SP was administered at a secondary site (i. p.) at the time of sperm immunization (subcutaneous). When SP components were tested individually in in vitro assays, all of them (prostate, seminal vesicle, and epididymis) suppressed mitogen-induced lymphocyte transformation. Prostatic fluid also inhibited complement-mediated hemolysis in a standard immune hemolytic assay. These data indicate that potent immunosuppressive factors are present at several locations within the male reproductive tract; these factors may serve to protect sperm from immunologic damage and prevent sensitization of females to sperm antigens after insemination.