Defensins, human neutrophil peptide (HNP) antibiotics, potently inhibited phospholipid/ Ca²⁺ protein kinase (protein kinase C, PKC) and phosphorylation of endogenous proteins from rat brains catalyzed by the enzyme. Of the three defensin peptides, HNP-2 appeared to be more potent than HNP-1 and HNP-3. Kinetic studies indicated that defensins inhibited PKC noncompetitively with respect to phosphatidylserine (a phospholipid cofactor), Ca²⁺ (an activator), ATP (a phosphoryl donor) and histone HI (a substrate protein) with K_i values ranging from 1.2 to 1.7μM. Defensins, unlike polymyxin B (another peptide inhibitor of PKC), did not inhibit the binding of [³H]phorbol 12,13-dibutyrate to PKC; however, defensins, like polymyxin B, inhibited the PKC activity stimulated by 12-O-tetradecanoylphorbol-13-acetate. Defensins had little or no effect on myosin light chain kinase (a calmodulin/Ca²⁺-dependent protein kinase) and the holoenzyme or catalytic subunit of cyclic AMP-dependent protein kinase, indicating a specificity of action of defensins. It is suggested that defensins, among the most potent peptide inhibitors of PKC so far identified, may have profound effects on functions of neutrophils and other mammalian cells, in addition to their well-recognized antimubial activities.