This article reviews possibilities for immunological contraception and its advantages for world population control. Current population growth estimates, particularly for the underdeveloped countries, indicate an effective means of fertility control that can be used massively with long-range utility is needed to avert an intolerable world wide population size by the end of the century. Reversible sterility by injection would be particularly useful in this situation. Antibody reactions in both males and females to both sexes' gonad tissues are being studied in an attempt to develop a suitable vaccine. Antibodies to testicular tissue and spermatoza have been found in higher quantities in infertile men than in fertile men, and presumably they interfere with spermatozal production and viability. These could be used in both men and women. In addition, antigens have been found in female cells which, when injected in females, may interfere with the estrous cycle or pregnancy. Because of the great advantages inherent in the immunological approach, these methods provide a challenging goal for further research.