['Retinal pigment epithelial (RPE) cells that enter the vitreous in pathologic conditions, such as retinal detachment, may proliferate and contribute to the formation of epiretinal membranes.', 'To study whether hyalocytes, endogenous vitreous cells, play a role in modulating the proliferation of RPE cells.', 'Cell proliferation was measured by tritiated thymidine incorporation in density-arrested human RPE cells after incubation with media that had been conditioned by cultured bovine hyalocytes. Preliminary characterization of inhibitory activity in hyalocyte-conditioned medium was performed, including blocking experiments with a neutralizing antibody to transforming growth factor-beta 2 (TGF-beta) and proliferation assays that used MV-1-Lu mink lung epithelial cells. Northern blots were done to asses hyalocyte expression of TGF-beta messenger RNA.', 'Hyalocyte-conditioned medium inhibited tritiated thymidine incorporation in RPE cells and MV-1-Lu mink lung epithelial cells in the presence or absence of serum or protease inhibitors. A portion of the inhibitory activity was neutralized by an antibody directed against TGF-beta. Northern blots of hyalocyte RNA demonstrated the presence of messenger RNA for TGF-beta 2. These data suggest that TGF-beta is responsible for a portion of the inhibitory activity secreted by hyalocytes. Additional inhibitory activity is attributable to one or more low-molecular-weight molecules distinct from TGF-beta.'] Do hyalocytes synthesize and secrete inhibitors of retinal pigment epithelial cell proliferation in vitro?