Several recent reports have confirmed the original postulate of McCullagh (1932) that the testis secretes a non-steroidal hormone which selectively inhibits pituitary FSH secretion. For example, extracts of bovine testis depress FSH levels in castrated male sheep (Lee, Keogh, de Kretser & Hudson, 1974) and FSH secretion in male rats is inhibited by human and bull seminal plasma (Franchimont, 1972; Franchimont, Chari, Hagelstein & Duraiswami, 1975). Inhibin activity has also been demonstrated in rete testis fluid (Setchell & Sirinathsinghji, 1972; Setchell & Jacks, 1974). Prompted in part by the fact that no known hormone is confined exclusively to one sex, we decided to test bovine ovarian extracts for inhibin-like activity. A preliminary account of this work has appeared (Hopkinson, Sturm, Daume, Fritze & Hirschhäuser, 1975).