A patient with the resistant ovary syndrome is reported. To evaluate the hypothesis that the hypogonadism might be the result of circulating antibodies to gonadotropin receptors or to an abnormal gonadotropin molecule, a series of clinical and laboratory studies was carried out. Administration of human menopausal gonadotropin had no effect on the serum estradiol level. The patient's serum did not affect follicle-stimulating hormone binding to a membrane preparation of monkey testes, suggesting the absence of antibodies to follicle-stimulating hormone receptors, nor did the patient's serum affect in vitro responsiveness of human granulosa cells to human menopausal gonadotropin. Unresponsiveness to exogenous gonadotropins, combined with anatomically normal follicular apparatus and the absence of serum antibodies to gonadotropin receptors, supports the concept of a gonadotropin receptor or a postreceptor defect.