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A simple, short procedure for the isolation of inhibin from bull seminal plasma has been described. Following conversion of the seminal plasma into acetone powder, it was subjected to gel filtration on Sephadex G-100 using 4 m urea\p=m-\0.05 m sodium acetate buffer pH 4.0 as the eluent. Further purification of the active fraction was achieved by 'straight elution analysis' using CM-cellulose. The final preparation (UX) has been shown to reduce the response of immature rat ovaries to human chorionic gonadotrophic hormone (HCG) stimulation in the bioassay system of Chari et al. (1976) and also been shown to suppress the immediate post-castration rise in serum gonadotrophin levels in immature male rats. Homogeneity of the isolated fraction has been assessed electrophoretically at different pH's above and below its isoelectric point. On the basis of gel filtration studies, electrophoresis and amino acid analysis, the molecular weight of UX has been estimated to be 19 000 daltons. The electrophoretic behaviour of UX at an alkaline pH has been discussed as a possible evidence for its existence as isohormones.

A bioassay for inhibin, based on a dose-dependent suppression of HCG-induced ovarian weight increase in intact, immature female rats, is described. The method is specific and has acceptable sensitivity and precision such that it lends itself to statistically valid quantitative assay of inhibin activity. Being a 24-hour assay, it is conveniently and rapidly performed as well as being a multiple bioassay for estimating potencies of various preparations.

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