This study was performed to examine effects of the overexpression of protein kinase C (PKC) isoforms (i.e., beta I, beta II, gamma, delta, eta, and zeta) on mitogen-activated protein (MAP) kinase (Erk-1 and -2) signaling and growth characteristics of NIH3T3 cells. Phorbol ester (PMA) activated endogenous and ectopically expressed PKC alpha, beta I, beta II, gamma, delta, epsilon, and eta. Overexpression of the examined PKC isoforms enhanced PMA-induced MAP kinase activation. Potentiation of MAP kinase activation was also observed upon stimulation of cells with platelet-derived growth factor (PDGF) although there was no indication for the activation PKC isoforms by PDGF. Inhibition of PKC blocked PMA- but not PDGF-induced MAP kinase activation. Thus, potentiation of PDGF-induced MAP kinase activation appears to be independent to PKC activity, while PMA-induced MAP kinase activation requires PKC activity. The ability of PKC isoforms to potentiate MAP kinase activation is not related to the growth characteristics of cells because individual PKC isoforms differentially regulated maximum density and proliferation of cells.