We investigated the biological effects of sex-steroid hormones, secreted from the corpus luteum and placenta, on the induction of mRNAs encoding macrophage colony-stimulating factor (MCSF) and c-fms proto-oncogene (MCSF receptor) in human endometrium. RNA was extracted from the placenta and endometrium of both pregnant and non-pregnant women, and Northern blot analysis was performed on poly(A)+ RNA using MCSF or c-fms proto-oncogene cDNA as the probe. Results showed: (1) that MCSF mRNA was expressed in the placenta and endometrium of the pregnant uterus, (2) that c-fms proto-oncogene mRNA was also expressed in the placenta and endometrium of the pregnant uterus, and (3) that exogenous sex-steroid hormones could induce the expression of MCSF and c-fms proto-oncogene mRNAs in the endometrium of non-pregnant women. These results indicate that sex-steroid hormones secreted by the corpus luteum and/or placenta influence endometrial and placental growth and differentiation via a mechanism of action involving local production of MCSF and its receptor.