Progesterone binding components were characterized and measured in the 0.4 M KCl nuclear extract of Premarin-primed human leiomyoma (LM) following progesterone injection (n = 5) using [3H]R5020 as the ligand. The receptor character of the binding was demonstrated by: high specificity for progesterone and R5020 binding, high saturable affinity (Kd approximately 2.5 nM) for R5020, sedimentation coefficient in high salt concentration (approximately 3 S), change of distribution of the receptor after progesterone administration, i.e. an increase in the nuclear and decrease in the cytosolic receptor levels. Similar results were demonstrated in the corresponding myometria and endometria. The characteristics of the nuclear progesterone receptor in all three tissues of the test group were similar to those of the cytosolic progesterone receptor in the test group and in the control group (no treatment before operation, n = 5). These results indicate that the nuclear progesterone receptor in human uterine LM shares similar characteristics to those of the myometrium and endometrium.