PKCdelta was revealed to make a homologous protein complex that shows a high protein kinase activity upon H(2)O(2) stimulation by expressing the enzymes having different epitope tags in COS-7 cells. The association of the endogenous PKCdelta in the cells was observed by sucrose density gradients. Analysis using the mutant replacing the tyrosine phosphorylation sites showed that PKCdelta is activated without tyrosine phosphorylation in the stimulated cells, and the time course of the activation was parallel with that of the complex formation. The binding sites were identified as the C1 and C2-like regions in the regulatory domain using a series of deletion mutants. The binding between the C1 and C2-like region fragments was induced by cell stimulation, whereas the association of the C1 region fragments by itself and that of the C2-like region fragments were observed even without stimulation. These results suggest that the protein complexes of PKCdelta through the association between the C1 and C2-like regions by different combinations are generated in the H(2)O(2)-treated cells, that may show an enhanced protein kinase activity.