A RIA for bovine (b) FSH-suppressing protein (FSP) was developed using an antiserum raised in a rabbit to purified 39-kDa bFSP, iodinated 35-kDa FSP as tracer, and purified 35-kDa bFSP as standard. Purified 35-kDa FSP was iodinated using the iodogen procedure, and the iodinated FSP was purified by dye affinity chromatography. After a logit log-dose transformation of the dose-response curves, parallel displacement lines were observed between 31-, 35-, and 39-kDa FSP, bovine follicular fluid, bovine granulosa cell culture medium, and medium from bovine granulosa cells stimulated with bFSH. The specificity of the assay was investigated by comparing the immunoassay levels of FSP with in vitro bioassay levels based on the ability of FSP/inhibin to suppress FSH in rat anterior pituitary cell cultures in fractions obtained throughout the purification procedure of FSP from bovine follicular fluid. This demonstrated that 1) the FSP immunoactivity was associated with in vitro bioactivity in all fractions of the purification procedure; 2) a number of inhibin-related and other proteins showed low (less than 0.5%) or nondetectable cross-reactivity in the RIA; and 3) the in vitro biological to immunological ratios for 31-, 35-, and 39-kDa FSP were similar, indicating that the RIA detects all forms of purified bFSP. The secretion of FSP by bovine granulosa cells in culture was investigated in the presence and absence of bFSH and bLH, respectively. FSP production was proportional to granulosa cell number and decreased from highest levels at 24 h to lowest levels at 96 h of culture. The addition of either bFSH or 8-bromo-cAMP to the culture medium stimulated FSP production by a factor of 2-3 at 48 and 72 h of culture, while the addition of bLH had no effect on FSP production. Theca interna tissue cultured under the same conditions did not produce FSP. In contrast to FSP, stimulation of bovine granulosa cells with bFSH or bLH had no effect on inhibin production during the 96 h of culture, while the addition of bFSH and bLH caused a stimulation of progesterone production at 48 and 72 h of culture. It is concluded that 1) the RIA described here is specific for all mol wt forms of bFSP; 2) FSP was secreted by bovine granulosa cells and not thecal cells in vitro; and 3) FSP secretion by bovine granulosa cells in vitro is regulated by bFSH and not bLH.