Under conditions of overwhelming infection of L and HeLa cells, with the New Jersey type of vesicular stomatitis virus (VSV), maximal infectivity titres were attained within 12 hours post-infection. Staining with anti-VSV fluorescent gamma globulin revealed within 4 to 6 hours after infection extensive accumulation of viral antigen in the cytoplasm. The fluorescent patches were of variable sizes and nuclei were not involved at any time. Prior to total destruction, cytoplasmic fusion occurred in L but not in HeLa cultures, resulting in the formation of multinucleated giant cells. Electron microscopy failed to detect any significant morphological changes in the cytoplasm of infected cells which might correspond to the observed immunofluorescent deposits. Efforts to locate such areas in cells rendered permeable to ferritin conjugated antibodies were unsuccessful. Within 4 to 6 hours after infection, virus accumulated at the cellular boundary or in cytoplasmic vacuoles. As a rule, only particles located at the cell periphery were readily tagged with ferritin antibody.