The calcium-dependent and phospholipid-dependent protein kinase C is partially purified (2000-fold) from chick oviduct cytosol by chromatography on DEAE-cellulose, Sephadex G150 and (after affinity binding of the enzyme to phosphatidylserine-diolein liposomes) on Bio-Gel A-0.5m. The enzyme is activated by phosphatidylserine and calcium ions. Diolein increases the affinity of the enzyme for both cofactors and can be replaced by the tumour-promoting phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate. Phosphatidylserine can be replaced by unsaturated long-chain fatty acids. Quercetin and phloretin inhibit the kinase reversibly competing with ATP. Several oviduct proteins are found to be phosphorylated by the partially purified enzyme. One of the substrates, a 78-kDa protein, appears to be ovotransferrin.