The development of viral antigen detectable by the fluorescent antibodytechnique and the production of infectious virus were studied iua Vero and E. Derm(NBL-6) cells infected with equine arteritis virus. In both cells the latent period lasted10 to 12 hr, followed by an exponential increase of infectious virus, and the peak Literwas reached 16 to 24 hr after infection. Viral antigen appeared as a small number ofgranules in the perinuclear region of the cytoplasm as early as l0hr after infection ina few cells. Fourteen to 16hr after infection, viral antigen filled the cytoplasm. At 24hr, cell rounding was found in fluoresced cells and some cells became detached from the glasswall. Viral antigen was detected exclusively in the cytoplasm. Although the above find-ings were common in both cells, the intensity of fluorescence was stronger in the E. Derm(NBL-6) cells than in the Vero cells.