Normal human melanocytes proliferate in vitro only in response to costimulation by at least two selected peptide growth factors. In the presence of only one mitogen, melanocytes become quiescent or die. These mitogens also enhance expression of differentiated functions, since in their presence the proliferating melanocytes become progressively more pigmented. To assess the intermediates participating in this dual response, we have determined the activated state of several known ligand-induced signal transducers. We demonstrate that hepatocyte growth factor/scatter factor, mast/stem-cell growth factor, basic fibroblast growth factor, and endothelin-1 induce phosphorylation of Ser133 within the KID domain of the cAMP-responsive element binding protein, a modification necessary for transcriptional activation of all members of this family of transcription factors, including also cAMP-responsive element modulator tau and activating transcription factor 1. The costimulation with synergistic growth factors prolonged the phosphorylated state and activity of the mitogen-activated protein kinase 2 cascade. cAMP-responsive element binding protein Ser133 phosphorylation in response to synergistic growth factors was due probably to the activation of p90RSK and, to a lesser extent, to p70S6K. Our findings support the concept that signals initiated at the cell surface converge on regulatory proteins that sustain both cell division and differentiation.