The immunology of gonadal steroids is reviewed. Characteristics of antibodies to steroids, use of antibodies for the localization of steroids in tissues and physiological studies using antibodies to steroids (inhibition of exogenously administered hormones, in vivo specificity of the steroid-antibody reaction, mechanism of action of antibodies, inhibition of endogenous circulating hormones and use of antibodies to study the role of hormones in various physiological and pathological processes) are the main topics. Therapeutic possibilities of immunization with steroids are many. Active immunization with estrogen conjugates could cause lasting anovulation. This condition could be reversed by the administration of synthetic estrogens not inhibited by antibodies. Also, antibodies to progesterone could be used to terminate unwanted pregnancies. Antibodies have unique advantages over other methods by which inactivation of a hormone is sought since antibodies have no biological activity other than by their capacity to inactivate the homologous hormone in the peripheral circulation.