A modified bovine testicular receptor was used to evaluate highly purified follicle-stimulating hormone (FSH) from a number of species. The particulate receptor obtained from adult bovine testes could be stored frozen or lyophilized for long periods without appreciable decrease in the binding of the ligand facility or in loss of specificity. The bovine testis receptor binds twice as much 125I-labelled ovine FSH as 125I-labelled human hormone. When FSH from different species was compared against NIH-FSH-S10, using various FSH ligands, the ovine hormone was clearly the most active, although many had comparable in-vivo biological potencies. The results suggest that there is probably some species specificity in the hormone-receptor interactions. As the ovine hormone is structurally closer to the bovine, from which the receptor was derived, it appears to have the highest activity in vitro. Marked differences in the biological activities of the different preparations between the human chorionic gonadotrophin-augmentation test and the in-vitro assays have been observed. In the in-vitro assays, all preparations, with the exception of the porcine hormone preparation, were less active and the ratio of bioassay/radioreceptor assay varied widely. In the radioreceptor assays, all FSH preparations except pregnant mare serum gonadotrophin (PMSG) showed parallel inhibition curves. The three different PMSG preparations examined gave inhibition lines that were parallel to each other.