When 3H-progesterone was incubated with the 273 000 g supernatant fraction from human uteri, labelled steroid was bound to a macromolecular component. Concomitant incubation with unlabelled corticosteroids, progesterone, testosterone and oestradiol-17β depressed binding of 3H-progesterone, whereas cholesterol and the synthetic progestins chlormadinone and medroxyprogesterone acetate did not competitively inhibit 3H-progesterone binding. These findings are consistent with the existence of a transcortin-like progesterone binder in uteri of lower species such as the rat, guinea pig and rabbit.