Protein kinase C (PKC) phosphorylated a synthetic peptide (CBP) that included the Thr-286 phosphorylation sequence and calmodulin binding domain of Ca2+/calmodulin-dependent protein kinase type II (CaM-kinase). Studies with a variety of truncated peptides suggested that the amino acid phosphorylated by PKC was Thr-286, the same amino acid that when autophosphorylated by Ca2+/calmodulin activation of CaM-kinase results in Ca2+/calmodulin-independent activity. These peptide studies also suggested that the C-terminal region of CBP is required to obtain maximal phosphorylation of Thr-286 by PKC. PKC also phosphorylated purified CaM-kinase from rat forebrain. Phosphopeptide analysis by one- and two-dimensional proteolytic maps of autophosphorylated CaM-kinase and CaM-kinase phosphorylated with PKC identified that there are both similar and unique sites phosphorylated. Phosphoamino acid analysis of CaM-kinase phosphorylated by PKC indicated that both Ser and Thr residues were phosphorylated. Even though Thr-286 of CaM-kinase appeared to be phosphorylated by PKC, no Ca2+/calmodulin-independent activity was detected, and, additionally, no significant change in Ca2+/CaM-dependent activation was detected. These results provide the first indication that these two important protein kinases may communicate directly through interenzyme phosphorylation.