Increased levels of maternal serum progesterone occur during the last stages of human gestation. The function of the high level of this steroid is unknown. The presence of a progesterone receptor in the placenta was investigated to determine whether progesterone action on the placenta might serve as one function of the high level of this steroid. Cytosol and nuclear fractions, derived from human placentae and fetal membranes, were examined for the presence of progesterone receptors by conducting exchange assays, using tritiated R 5020 (17,21-dimethyl-19-norpregna-4,9-diene-3,20-dione) as the radiolabelled ligand. High-affinity, low-capacity binding, characteristic of steroid receptors, was estimated as the difference between binding of radiolabelled ligand in the presence of no unlabelled ligand and that in the presence of a 100-fold excess of unlabelled ligand. These exchange assays were conducted during a 24-hour period at 0 degrees C, to allow maximal stability of the receptor, and during a 3-hour period at 20 degrees C, to allow the rapid exchange of radiolabelled ligand for any bound endogenous progesterone. The assays of all fractions showed no specific binding of the R 5020, thus indicating the absence of progesterone receptors in the cytosol and nuclei of the human placenta and the fetal membranes.