The expression and secretion of different isoforms of transforming growth factor-β (TGFβ) were examined in cultured transformed and nontransformed human retinal pigment epithelial (RPE) cells. Transformed RPE cells were found to express high levels of TGFβ1 mRNA, low levels of TGFβ3 mRNA but no detectable TGFβ2 mRNA. If the cells were grown under serum-free conditions the expression of TGFβ increased. The mRNA expression was accompanied by secretion of TGFβ1 (but not TGFβ2) protein into the culture media. By comparison, nontransformed RPE cells were found to secrete similar amounts of TGFβ as transformed cells but predominantly secreted TGFβ2. The secretion of TGFβ from both transformed and nontransformed RPE cells increased if the cells were grown without serum. In conclusion, the results show that TGFβ is expressed and secreted by transformed and nontransformed human RPE cells and that this expression and secretion are regulated by the presence or absence of exogenous factors.