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A breeding problem of sheep maintained on pastures dominated by the "Dwal^anup" strain of subterranean clover, first reported by Bennetts (1944), •was shown hy Bennetts. I'nderwood and Shier (1946) to be characterized by three main clinical nianifestaiions: dystocia. uterine prolapse and female infertility. These workers reported the frequent oeeurreiiee of cystic glandular hyperplasia of tlie endometrium of ewes in "affected" flocks, and they produced similar chanf?es in tlie cudomctrinm of guinea-pifjs fed wholly on either fresh green, airdried, or artificially dried clover from an "affected" area. These findings, together with the demonstration by Bennetts (1946Bennetts ( , 1947 of s(|namons metaplasia in the sex organs of castrated male sheep depastured on "affected" herbage, provided strong presumptive evidence of the presence in tbe clover of a potent oestrogenic substance, or its precursor. Bennetts et al. (1946) did not, however, notice any abnormalities in the oestrons cycle of either the sheep or the guinea-pigs, such as might be expected in animals receiving high and regular intakes of a normal oestrogen. Tt became important, therefore, to obtain further evidence as to the oestrogenic nature of the potent factor in the clover and its physical and chemical properties.The present paper is the first report of this work. Evidence is presented to show that there is an oestrogenic factor in early subterranean clover which is ether-solnble and active wben given by moutb, producing physiolf^ical effects similar to known oestrogens, namely, (1) early vaginal opening in immature mice, (2) vaginal cornification in mice, (:J) uterine Itypertrophy in mice and guinea-pigs, (4) cystic endometrium in guinea-pigs uudei-conditions of longer administration.Some of the chemical properties of the active compound are also presented. Farther work on the chemical characterization of the active principle is now proceeding and will be published later.Considerable progress has also beeu made in developing an efficient means of extracting the activity from plant materials and making accurate assays of potency. Tbe results of tbis work will be published shortly by Robinson (1947).