Sinusoidal Ito cells (stellate or fat-storing cells) undergo excessive cellular proliferation before the establishment and progression of hepatic fibrosis and cirrhosis. Retinoic acid and transforming growth factor β (TGFβ) both inhibit Ito-cell [3H]thymidine incorporation in serum-containing media. Serum-induced mitogenicity was dependent on platelet-derived growth factor (PDGF). Additionally, pre-treatment of Ito cells with retinoic acid and TGFβ blocked PDGF-induced cell proliferation. TGFβ, but not retinoic acid, diminished PDGF-receptor and smooth-muscle α-actin abundance.