The biologic (B) and immunologic (I) properties of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were studied in three commercial urinary gonadotropin preparations and in the first international standard preparation of human urinary gonadotropins before and after fractionation by isoelectrofocusing (IEF). Significant differences were found in the IEF profiles of both bioactive and immunoreactive LH and FSH and in the B/I ratios of the preparations studied. The observed differences in the molecular composition of LH and FSH seem to be attributable to the purification procedures employed. The possible influence of these differences on the in vivo potencies, circulating half-lives, and clinical effects of gonadotropin preparations are discussed.