Protein kinase C (PKC), a protein phosphorylating enzyme, is characterized by its need for an acidic phospholipid and for activators such as Ca2+ and diacylglycerol. The substrate commonly used in experiments with PKC is a basic protein, histone III-S, which needs the activators mentioned. However, protamine, a natural basic substrate for PKC, does not require the presence of cofactor/activator. We report here that protamine can induce the autophosphorylation of PKC in the absence of any PKC-cofactor or activator; this may represent a possible mechanism of cofactor-independent phosphorylation of this protein. It was investigated if protamine itself can act as a PKC-activator and stimulate histone phosphorylation in the manner of Ca2+ and phospholipids. Experiments however showed that protamine is not a general effector of PKC. On the contrary, histone stimulated PKC-mediated protamine phosphorylation and protamine-induced PKC-autophosphorylation. Histone alone did not induce PKC-autophosphorylation. Kinetic studies suggest that histone increases the maximal velocity (Vmax) of protamine kinase activity of PKC without affecting the affinity (Km). Other polycationic proteins such as polyarginine serine and polyarginine tyrosine were not found to influence PKC-mediated protamine phosphorylation, indicating that the observed effects are specific to histone, and are not general for all polycationic proteins. These results suggest that histone can modulate the protamine kinase activity of PKC by stimulating protamine-induced PKC-autophosphorylation.