Epidermal growth factor stimulates phosphorylation of ribosomal protein S6 in serum-starved Swiss 3T3 cells, leading to the formation of highly phosphorylated derivatives containing 4-5 phosphates. Two-dimensional analysis of tryptic phosphopeptides of S6 shows an identical pattern to the ones obtained previously in other cells in response to insulin and the tumor promoting phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate. This suggests a common intracellular mediator of S6 phosphorylation by different growth promoting agents. It is proposed that the potential mediator of this phosphorylation is the Ca2+-independent, cAMP-independent protein kinase, protease activated kinase II, as shown by the extent of phosphorylation and the tryptic phosphopeptide maps of S6 with highly purified enzyme [(1983) J. Biol. Chem. 258, 13998-14002].