During the assembly of the nucleocapsid of the hepatitis B virus a protein kinase, probably of cellular origin, is encapsidated. This enzyme phosphorylates serine residue(s) localized within the lumen of the particle. By using purified, liver-derived core particles, we characterized the protein kinase activity in the presence of different ions and inhibitors. Controls were performed with cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) and recombinant core particles. We showed that the endogenous protein kinase of the core particles was not inhibited by H89, a specific inhibitor of PKA. Staurosporine, a selective inhibitor of PKC inhibited the endogenous kinase activity only within the first minutes of the reaction. In contrast, quercetine, a selective inhibitor of the protein kinase M (PKM) did not inhibit during the first minutes but inhibited efficiently during later phases of incubation. PKM represents an enzymatically active proteolytic fragment of PKC. These results suggest that PKC is encapsidated into human core particles and is converted to PKM during the in vitro reaction. This conclusion implies the association of a protease activity localized with the HBV nucleocapsid inside liver-derived core particles.