We have previously shown that rat liver epithelial cells were more sensitive to TGF-β 1 when they were transfected with c-fos cDNA. We analyzed the production of TGF-β and TGF-β 1 binding proteins in transfected and parental cells. TGF-β-like activity released in the medium was reduced in c-fos expressing cells. TGF-β 1 binding sites were more numerous in transfected cells ( 3). Cross-linking studies confirmed that c-fos transfected cells showed increased binding of 1 2 5 I-TGF-β 1 to membrane binding sites corresponding to type I, II and III receptors. Transfected cells internalized and degraded 1 2 5 I-TGF-β 1 more rapidly than parental cells. TGF-β 1 incubation rapidly down-regulated the receptors. In parental cells, the down-regulation was total, while in transfected cells, a few binding proteins could still be detected. The c-fos cell line is an interesting tool in analysing the mechanism of action of TGF-β.