As part of an inquiry into factors that determine the virulence of fixed rabies virus, mouse neuroblastoma cells were infected in culture with high virulence and low virulence strains of Flury HEP virus. Low virulence virus infection differed from high virulence virus infection in its more rapid production of progeny virus in the early cycles of virus infection as shown by the number of extracellular virus particles and the infectivity of the supernatant fluid, its earlier development of viral antigens on the cell surface and its earlier and more severe morphologic alteration of the cell surface. Where applicable, the differences were corroborated by scanning and transmission electron microscopy of the infected cells using the critical point drying technique on whole cells. The number of cells susceptible to complement-dependent immunolysis was almost proportional to the number of cells that were surface antigen-positive regardless of the strain of the virus used. Implications of the difference in the kinetics of virus production and of the development of surface antigens between low and high virulence strains are discussed.