The effects of a neonatal calf diarrhea virus on cell cultures were investigated. Bovine embryonic kidney cell cultures were the most satisfactory for production of virus. Cytoplasmic changes detected after inoculation with a high multiplicity of virus were: 1) cytoplasmic vacuoles; 2) some eosinophilic cytoplasmic inclusions; and 3) some degeneration of cells and detachment from the monolayer. Cultures stained with fluorescein-labeled antibody showed cytoplasmic fluorescence as early as four hr after infection with the maximum fluorescence at five days. No cross reactions were observed between the neonatal calf diarrhea virus and reovirus type 1 or type 3 by the fluorescent antibody technique. Plaques were small and were not produced consistently. The optimal adsorption time was one to two hr. The maximum titer was reached at 18 hr, with the cell-associated titer remaining higher than the cell-free titer until that time. An interferon was produced by cultures infected with either ultraviolet-inactivated or untreated virus.