Results of this investigation indicated that the infectivity titre of mouse hepatitis virus decreased after 6 hour incubation at 37°. Nutrient fluids from infected macrophage cultures retained pathogenicity in vivo and the ability to form polykaryons in vitro. Electron microscope studies of polykaryons formed from infected macrophage cultures showed the presence of virus-like particles in the cytoplasm. From the study, it was concluded that polykaryon formation in infected macrophage cultures resulted from an active viral agent.