Abstract The effects of inhibin purified from bovine follicular fluid (bFF) and a charcoal-treated bFF preparation were investigated in several inhibin in vitro systems based on the use of pituitary cells in culture. FSH, LH, TSH and PRL were determined in the medium and cell extracts before and after a 4 h LHRH stimulation test. Both pure inhibin and bFF preparations markedly inhibited the basal release, cell content and LHRH-stimulated release of FSH in a parallel dose-dependent manner with minor or negligible effects on LH, TSH and PRL. Using parallel line bioassay statistics the inhibin activities of the purified inhibin preparations in the various in vitro systems were calculated with the charcoal-treated bFF as standard. Significantly higher inhibin values were obtained using the basal release procedure than with the cell content or LHRH-stimulated release procedures. This difference was influenced by the length of time the inhibin preparations were in culture. The highly purified preparations showed no signs of cytotoxicity in culture as assessed by a 51Cr release test. It is concluded that purified bFF inhibin is specific in suppressing pituitary FSH and not LH, TSH or PRL. The observation that purified bFF inhibin is more active in the basal release procedure is attributed to a loss of activity of the bFF standard in this system in contrast to that observed in either the cell content or LHRH-stimulated release procedure.