The role of transforming growth factor-beta 1 (TGF-beta 1) in communication between human endometrial carcinoma (EC) cells and normal endometrial stromal cells (NSC) was investigated using a cell culture model. Serum-free conditioned medium (CMe) from EC cells (RL95-2, HEC1A) inhibited the proliferation (cells per colony < 50% of control; mitotic index 25-50% of control) of NSC. In contrast, NSC-conditioned medium (CMn) stimulated the proliferation of EC cells, but inhibited the growth of NSC. The proliferation of EC cells was stimulated by the range of dilutions of CMe which inhibited the proliferation of NSC. Using confocal microscopy and a monoclonal antibody, TGF-beta 1, a known product of differentiation in the female reproductive tract, was localized to the cytoplasm of NSC and EC cells. Using a protein slot-blot chemiluminescence method, secreted TGF-beta 1 was detected in serum-free medium conditioned by the growth of NSC and EC cells. TGF-beta 1 antibody-neutralized CMe or CMn stimulated the proliferation of both NSC and EC cells. This study suggests that endometrial carcinoma-stromal cell interactions involve autocrine-paracrine signaling pathways, and that TGF-beta 1 protein is one mediator of such interactions.