Cardiac sarcolemma proteins were phosphorylated by exogenous Ca2+-activated phospholipid-dependent protein kinase (protein kinase C). The phosphorylation reactions were absolutely dependent on the simultaneous presence of Ca2+ and phosphatidylserine. Phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, sphingomyelin, and phosphatidic acid were ineffective in supporting protein kinase C-catalyzed membrane phosphorylation. The reactions were not stimulated by diolein. In contrast, diolein inhibited phosphatidylserine-stimulated phosphorylation at all calcium concentrations tested. The major substrates for protein kinase C in cardiac membranes were peptides of 88,000, 51,000, 42,000 daltons, and the peptide known as phospholamban (Mr = 27,000 or 11,000 depending on sample preparation). Phosphorylation of phospholamban by protein kinase C was additive with that catalyzed by membrane-bound or exogenous cyclic AMP-dependent protein kinase and with Ca2+-calmodulin-dependent protein kinase. The results suggest that protein kinase C might have a role in the regulation of cardiac membrane phosphorylation by beta-adrenergic and muscarinic cholinergic agonists.