A specific complex of protein-receptor with hormone, which had a sedimentation coefficient 3.8S, was formed in cell nuclei on incubation of endometrial tissue from pregnant woman with 3H-progestrone at 37 degrees C in vitro. In cell cytoplasm of endometrium the progesterone-protein complex was observed which sedimented at 6.8S and 4.2S. Progesterone was accumulated mainly in nuclei; lower concentrations of the hormone were found in cell cytoplasm of endometrium. Fractions of mitochondria, microsomes and ribosomes also bound the hormone but less distinct as compared with the nuclei. The formation of cytoplasmic complex of progesterone with protein-receptor occurred on incubation of endometrial cytoplasmic fraction with 3h-progesterone at 4 degrees C.