We report that TGFα induces activation of the p90 ribosomal S kinase (RSK), which results in the phosphorylation of rat C/EBPβ on Ser-105 and of mouse C/EBPβ on Thr-217 and concomitantly stimulates proliferation in differentiated hepatocytes. Moreover, C/EBPβ^−/− mouse hepatocytes respond to TGFα when wild-type C/EBPβ is reexpressed, whereas they remain refractory to the growth effect of TGFα when expressing phosphoacceptor mutants rat C/EBPβ Ala-105 or mouse C/EBPβ Ala-217. In contrast, C/EBPβ^−/− hepatocytes expressing the phosphorylation mimic mutants, rat C/EBPβ Asp-105 or mouse C/EBPβ Glu-217, exhibited marked proliferation in the absence of TGFα. Thus, a site-specific phosphorylation of the transcription factor C/EBPβ is critical for hepatocyte proliferation induced by TGFα and other stimuli that activate RSK.