A review of the immunological implications in reproductive physiology is presented. Although attempts have been made to ascribe the antigenicity of semen to individual components, it has not been possible to isolate the human semen antigen responsible for infertility. In monkeys total ejaculates and seminal plasma have shown higher antigenicity than washed spermatozoa. In bulls some evidence of such antigens have been found in the seminal plasma. They are iron-binding proteins resembling lactoferrin. Most investigators have found no evidence for any participation of the ABO blood group antigens in cases of sterility. On the surface of human spermatozoa histo-incompatibility antigens have been detected. Transplantation antigens may be related to sterility. However, an immulogic tolerance of the maternal organism exists against the genetically foreign fetal tissue. Autoimmune spermagglutinating antibodies have been detected in the sera and in the seminal plasma of males with sterility. An obstruction of the seminal pathways may facilitate the production of such antibodies against retained sperm. Isoimmunity in females against seminal components has been shown in cases of sterility; however, fertile women have also been shown to have such conditions. In a group of infertile women spermagglutination activity was detected in 7.5% of cases. In another series of 46 cases with primary unexplained infertility agglutinating antibodies were found in 17.4%. Other investigators have also reported higher rates than the authors. The sperm immobilization test seems to be more sensitive than the agglutination test. No sera were found positive with both tests. With immunofluorescent techniques humoral sperm antibodies have been found to be the IgM and IgG fractions. Each acts on a different part of the spermatozoa. The only promising therapy against humoral sperm antibodies is avoidance of sperm contact over a long period of time. Reported results have been conflicting. Cortisone did not improve the result. Desensitization or immunosuppresive efforts are of uncertain value.