Recently, a model for early pregnancy in the rhesus monkey was developed which involved administration of a 10-day treatment regimen of human chorionic gonadotropin (hCG). The current studies were designed to determine whether monkeys produced antibodies in response to this hCG regimen and, if so, to characterize the specificity of the antibodies and their effects on subsequent menstrual cycles. Based on Sephadex chromatography and immunoglobulin precipitation with protein A, serum collected from hCG-treated monkeys contained binding components for 125I-hCG. Binding was specific for human gonadotropins (hCG, human luteinizing hormone [LH], human follicle-stimulating hormone [FSH]) and did not cross-react with macaque LH/FSH. Shortening the hCG regimen to less than or equal to 6 days did not result in significant binding. The 10-day hCG regimen did not alter subsequent menstrual cycles, but attenuated 17 beta-estradiol production in response to human menopausal gonadotropin. In conclusion, the 10-day regimen of hCG treatments used to simulate early pregnancy in rhesus monkeys induced the production of serum components with characteristics of hCG antibodies. These antibodies did not alter the regulation of the menstrual cycle, but attenuated the response to exogenous human gonadotropins.