Abstract

C-type viruses were identified by electron microscopy in muramidase-producing transplantable leukemic cells of BALB/c mice. Neoplastic cells contained peroxidase-positive cytoplasmic granules and consisted of atypical monocytic and granulocytic precursors, undifferentiated stem cells, and histiocytes. Viral particles were present in all tumors studied and were principally of the intracisternal A and immature C types. Exceptionally, mature C-type virions were noted in large cytoplasmic vacuoles of tumor cells, in bone marrow megakaryocytes, and in extracellular spaces. Unusual cytoplasmic structures that may represent structural precursors of viral nucleoprotein assembly were also observed in some cells. Bone marrow cells from animals treated with cell-free filtrates contained numerous cylindrical inclusions, several of which demonstrated viral budding. Complement fixation and mouse embryo cytopathogenicity tests on tumor extracts identified the viral particles as members of the murine leukemia-sarcoma group and demonstrated high titers of infectivity.