Human epidermal growth factor (EGF) and rat recombinant transforming growth factor alpha (TGFa) stimulated in a dose dependent manner the proliferation of rat liver fat storing cells (FSC) grown as primary culture in Dulbecco's modification of Eagles medium (DMEM) with 0.5% fetal calf serum (FCS). Cell proliferation was measured by DNA-content, [3H]thymidine- and bromodeoxyuridine-incorporation, respectively. Half maximal stimulation of [3H]thymidine incorporation was seen with 8ng/ml EGF and 7 ng/ml TGFa, respectively. In a radioreceptor assay using [125I]EGF as ligand TFGa binds with the same affinity as EGF to EGF-cell surface receptors on FSC. Eight ng/ml TGFa was required to produce half-maximal inhibition of [125I]EGF binding. If transforming growth factor beta (TGF beta) was added simultaneously with EGF or TGFa, FSC proliferation was inhibited strongly.