A fluorescent derivative of human chorionic gonadotropin (hCG) for binding studies was prepared by conjugation of hightly purified hCG with fluorescein isothiocynate (FITC-hCG) in a molar ratio of 3:1. The resulting FITC-hCG conjugate retained full cross-reactivity with antibodies directed against intact hCG, the hCG beta-subunit, the carboxyl-terminal peptide sequence of beta-hCG. The fluorescent hormone retained binding activity similar to that of native hCG, and the capacity to induce progesterone production from primate corpora luteal cells in vitro. After intravenous administration to female rhesus monkeys, FITC-hCG was localized within the theca cell layer of certain antral follicles, including the dominant follicle, but no fluorescence was noted around preantral follicles. Areas of fluorescence were also localized to discrete groups of interstitial gland cells in the stroma. The degree of fluorescence associated with these cells was indistinguishable from that observed in the thecal cells. The development of a fluorescent derivative that is biologically and immunologically similar to native hCG has permitted differential microscopic localization of luteinizing hormone/hCG uptake in the preovulatory ovary.