Newcastle disease virus infection of Ehrlich ascites tumor cells resulted, after a period of time, in the appearance of intracellular viral antigen which could be demonstrated by the fluorescent antibody technique. This antigen appeared in the cytoplasm of infected cells only after inoculation of cell-virus mixtures into the peritoneal cavities of mice. The latent period prior to the appearance of antigen depended inversely on the number of viral particles adsorbed onto the cells prior to inoculation. The final intensity of staining appeared not to be proportionate to the number of viral particles adsorbed to each cell. The appearance of this antigen was not correlated with a rise of titer of infectious, hemagglutinating, or complement-fixing virus. Viral antigen was demonstrated on the surface of tumor cells after adsorption of NDV onto these cells at 0 degrees C. At appropriate virus:cell ratios, antigen was noted to disappear from the surface at 37 degrees C. in vitro, and in vivo, in the absence of demonstrable elution of virus. The appearance of intracellular viral antigen could not be detected in vitro when tumor cell-NDV mixtures were incubated at 37 degrees C., even when an average of 1550 "infectious particles" had adsorbed to each cell.