Transforming growth factor (TGF)-beta is known to increase mesangial cell (MC) matrix; however, its possible role on MC proliferation is controversial. We therefore studied the influence of TGF-beta on MC proliferation in culture and evaluated its effect on the platelet-derived growth factor (PDGF) B-chain as well as the expression of the PDGF beta-receptor. TGF-beta (1 ng/ml) increases MC DNA synthesis by approximately threefold after 48 h of incubation. TGF-beta-induced MC proliferation was also confirmed by cell counts. A neutralizing anti-TGF-beta antibody completely blocked this growth promoting activity. The levels of PDGF beta-receptor steady-state mRNA were increased by TGF-beta at 48 h. This was associated with an increase in receptor density per cell as measured by receptor kinetic studies. PDGF B-chain mRNA was also increased by TGF-beta at 48 h. A neutralizing anti-PDGF B-antibody causes no reduction of TGF-beta-induced DNA synthesis; however, suramin completely inhibited the mitogenic effect of TGF-beta. We conclude that TGF-beta stimulates MC growth in long-term culture, a process in which upregulation of the PDGF beta-receptor and enhanced synthesis of PDGF B-chain might be involved.