It has been proposed that H2O2 increases tyrosine phosphorylation of cellular proteins by inhibiting protein-tyrosine phosphatase through oxidation of the cysteine residue of the enzyme essential for its catalytic activity. Tyrosine phosphorylation of the delta isoform of protein kinase C (PKC) was induced by H2O2 in CHO and COS-7 cells, H2O2 also induced activation of mitogen-activated protein kinase. Vanadate and molybdate, which inhibit protein-tyrosine phosphatase by binding to its active site, did not induce tyrosine phosphorylation of PKC delta, but enhanced H2O2-induced tyrosine phosphorylation of PKC delta in the cell. The oxoanions, however, generated the active form of mitogen-activated protein kinase. Another protein-tyrosine phosphatase inhibitor, phenylarsine oxide, which bridges the thiol residues of the enzyme, induced tyrosine phosphorylation of PKC delta, and the reaction was enhanced by vanadate, These results suggest that inhibition of protein-tyrosine phosphatase is insufficient for induction of tyrosine phosphorylation of PKC delta in the cells, and that presumably activation of protein-tyrosine kinase may be essential for tyrosine phosphorylation of the PKC isoform.