Protein kinase P (PK-P) activated by histones or certain other basic compounds has been purified previously from yeast [Yanagita, Y., Abdel-Ghany, M., Raden, D., Nelson, N. & Racker, E. (1987) Proc. Natl. Acad. Sci. USA 84, 925-929]. It is shown here that PK-P is present in solubilized membranes of A-431 carcinoma cells where it changes the epidermal growth factor (EGF) receptor kinase activity. Polylysine, a weak PK-P activator, inhibited the autophosphorylation of the EGF receptor both in the absence and presence of EGF. Increased PK-P activity induced by histone 1, a potent activator, gave rise to increased autophosphorylation of the EGF receptor as well as phosphorylation at tyrosine residues of numerous other endogenous membrane components. The stimulation by histone was particularly striking in the presence of EGF. A similar stimulation was achieved with polylysine and EGF on addition of yeast PK-P. However, addition of yeast PK-P in the presence of histone 1 markedly inhibited the EGF-stimulated phosphorylation of endogenous membrane proteins. We conclude from these results that the effect of PK-P on the EGF receptor takes place in three phases: at low levels PK-P inhibits the autophosphorylation, at intermediate levels it stimulates the autophosphorylation as well as the EGF-dependent phosphorylation of numerous other membrane proteins, and at high levels it inhibits the phosphorylation of these proteins.