Protein kinase C (PKC) isoforms, alpha, betaI, and gamma of cPKC subgroup, delta and epsilon of nPKC subgroup, and zeta of aPKC subgroup, were tyrosine phosphorylated in COS-7 cells in response to H2O2. These isoforms isolated from the H2O2-treated cells showed enhanced enzyme activity to various extents. The enzymes, PKC alpha and delta, recovered from the cells were independent of lipid cofactors for their catalytic activity. Analysis of mutated molecules of PKC delta showed that tyrosine residues, which are conserved in the catalytic domain of the PKC family, are critical for PKC activation induced by H2O2. These results suggest that PKC isoforms can be activated through tyrosine phosphorylation in a manner unrelated to receptor-coupled hydrolysis of inositol phospholipids.