Epidermal growth factor (EGF) or phorbol 12-myristate 13-acetate (PMA) stimulated mouse embryo palate mesenchyme (MEPM) cells to incorporate [32P]O(3-)4 into a protein with an apparent molecular weight of 80 kDa, in vitro. Agents known to elevate intracellular levels of cyclic AMP did not stimulate phosphorylation of this phosphoprotein. Since there is a significant amount of evidence obtained with other cells indicating that phosphorylation of such an 80-kDa phosphoprotein reflects specifically the activation of protein kinase C in response to PMA and other agents, including mitogens, these findings raise the possibility that EGF may activate protein kinase C in MEPM cells.