Retinal pigment epithelial (RPE) cells transdifferentiate in culture, a transition which is accompanied by a shift in biological activity. The present study investigates whether transforming growth factor (TGF)-β has the same effects on morphologically transformed RPE cells that it has on primary RPE cells. It also evaluates the autocrine and paracrine activities of TGF-βs synthesized by RPE cells as well as the anti-TGF-β effect of mannose-6-phosphate (M-6-P). RPE cells were subcultured at the sixth passage to induce morphological change. The effect of second passaged RPE-conditioned medium (CM) on DNA synthesis was evaluated by the incorporation of ³H-thymidine in rabbit subconjunctival fibroblasts (SCFs) and primary RPE cells. The presence of TGF-β in RPE-CM was determined using immunoblotting analysis. And the inhibitory effect of M-6-P on cell proliferation mediated by RPE-CM was also analyzed using ³H-thymidine incorporation into DNA. TGF-β1, TGF-β2, and TGF-β3 inhibited the proliferation of the primary cultures of RPE cells in a dose-dependent manner, but the spindle-shaped sixth passaged RPE cells were not inhibited by these growth factors. The medium conditioned by RPE cells stimulated the proliferation of SCFs and inhibited the proliferation of primary RPE cells, in a manner similar to TGF-β. When this medium was precipitated with either anti-TGF-β1, anti-TGF-β2, or anti-TGF-β3 antibodies, all three TGF-βs, with an apparent molecular size of 25 kDa, were detected. Mannose-6-phosphate significantly blocked the effect of RPE-CM on cell proliferation. These findings indicate that RPE cells produce biologically functional TGF-βs and that M-6-P can block the inhibitory effect of RPE-CM on cell proliferation.