We previously reported that recombinant interferon-gamma (IFN-gamma) induces HLA-DR (human lymphocyte antigen) molecules of the major histocompatibility complex in human endometrial epithelial cells in vitro. We now report that IFN-gamma inhibits the proliferation of human endometrial epithelial cells and a human endometrial carcinoma cell line (EnCa101AE). Human endometrial epithelial cells expressed HLA-DR molecules and underwent morphological changes when exposed to IFN. Furthermore, the proliferation of these cells, as evidenced by nuclear labeling of bromodeoxyuridine (an analog of thymidine that is incorporated into cells in S phase), was markedly reduced, in a dose-dependent manner, by IFN-gamma. IFN-gamma induced HLA-DR expression, morphological changes, shedding from the substratum, and cell death in EnCa101AE cells. In addition, cell number and the numbers of bromodeoxyuridine-, Ki-67 (a nuclear marker of proliferation)-, and MPM-2 (a marker of mitotic cells)-positive cells were markedly lower in the EnCa101AE cultures treated with IFN-gamma than those in control cultures. The cytostatic and HLA-DR-inducing effects of IFN-gamma could be abrogated by neutralization with a polyclonal antibody, and IFN-gamma effects were reversible within days after its withdrawal. These findings indicate that IFN-gamma inhibits proliferation of human endometrial epithelial cells and suggest that this factor may locally regulate the proliferation of these epithelial cells in vivo.