The nuclear polyhedrosis virus of the silkworm B. mori was obtained by dissolution of polyhedra at pH 10,5. It was shown that the virus particles contain a protease, which cleaves a few virion polypeptides during the dissolution of polyhedra under alkaline conditions. The proteolytic activity is inactivated by polyhedra treatment with phenylmethylsulfonylfluoride, diisopropylfluorophosphate, HgCl2 and at 80 degrees. Sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed 18 virion polypeptides with molecular weights ranging between 10000-110000. It was assumed that the virion includes the polyhedral protein and its fragments. A possible role of the polyhedral protein and its fragments in the virion is discussed.