The study was undertaken to identify and characterize insulin-like growth factor I (IGF-I) receptors in human endometrial stromal cells in culture and to examine whether these receptors are modulated by estradiol (E2) and/or progesterone (P). We found that partially purified plasma membrane proteins from these cells contain specific high-affinity binding sites for IGF-I (10 fmol/micrograms protein). Chemical cross-linking with 125I-labeled IGF-I and autophosphorylation with [32P]ATP-labeled proteins of relative molecular weight 135,000 and 95,000 correspond to the known Mr values of the alpha- and the beta-subunits of IGF-I receptors. Receptor autophosphorylation and phosphorylation of the substrate poly(Glu,Na4Tyr1) was stimulated in vitro by IGF-I (half-maximally at 1 nM, maximally at 100 nM). After stimulation of intact cells with IGF-I (5 nM) and subsequent partial purification of receptors in the presence of phosphatase inhibitors, a 2.5- to 3.6-fold stimulation of the kinase activity toward poly(Glu,Na4Tyr1) was found. Preincubation of the cells for 16 h with E2, P, and E2 + P did not modify the IGF-I binding characteristics nor the effect of IGF-I (5 nM) on tyrosine kinase stimulation in intact cells. This suggests that, in isolated humans, endometrial cell modulation of IGF-I receptor function by estrogen and P does not occur.