Specific binding of progesterone in chicken oviduct (Sherman, Corvol & O'Malley, 1970) and in the rat and rabbit uterus (McGuire & DeDella, 1971; McGuire & Bariso, 1972) has been demonstrated (but see Milgrom & Baulieu, 1970). Evidence for the binding of progesterone in the human endometrium has also been reported (Wiest & Rao, 1971). The work presented here demonstrates the existence of a progesterone binder in the cytosol fraction of the human myometrium. Tissues were removed during deliveries by Caesarean section, and placed in Krebs—Ringer bicarbonate medium (Batra & Daniel, 1970) at 25 °C. After dissecting away the endometrium, the myometrium was homogenized in sucrose—histidine solution (pH 7·4) and centrifuged as previously described (Batra, 1973). The supernatant was obtained after centrifugation at 110000g. Samples (0·5 ml) of supernatant (about 2 mg protein) were incubated at 4 °C with 2 × 10-9m-[3H]progesterone (0·05 μCi) in the