A protein which satisfies all the biological criteria which are characteristic of inhibin has been isolated from a gonadal source. The purification and characterization of inhibin and the use of the purified material to raise antibodies, the use of inhibin and said antisera in a quantative radioimmunoassay, and applications in vitro and in vivo of inhibin and antibody to inhibin, are described.There is provided a purified protein, inhibin, characterised in thata. the apparent molecular weight as determined by SDS-PAGE is 56,000.+-.1,000b. the isoelectric point is in the range 6.9-7.3c. the protein can bind specifically to Concanavalin A-Sepharosed the protein consists of two sub-units, characterized in thati. their apparent molecular weights as determined by SDS-PAGE are 44,000.+-.3,000 and 14,000.+-.2,000 respectively.ii. the isoelectric point of the 44,000 molecular weight sub-unit is in the range 6.0-7.0iii. the N-terminal amino acid sequences of the two sub-units are as described hereine. the protein can suppress follicle stimulating hormone (FSH) but not luteinising hormone (LH), thyroid stimulating hormone or prolactin in an in vitro bioassay systemf. the protein can be labeled with radioactive iodine.There is also provided a method for isolating and purifying inhibin from mammalian ovarian follicular fluid, characterised bya. one or more gel permeation chromatography steps;b. one or more reversed-phase high performance liquid chromatography steps;c. one or more preparative polyacrylamide gel electrophoresis steps;d. Electrophoretic elution of the purified inhibin.