Tunicamycin showed antiviral activity in the agar diffusion, cytopathic effect suppression and plaque reduction tests against the multiplication of Newcastle disease virus (NDV) in cultured chick embryo fibroblasts. It also inhibited virus multiplication in embryonated eggs. The antiviral activity was observed whenever the antibiotic was added during a one-step growth cycle of NDV. Reversal of the antiviral activity of tunicamycin was observed after removal of the antibiotic, but treatment for more than 2 hours permitted only slight recovery of the virus growth. Static cells were insensitive to tunicamycin, but the antibiotic effected on growing cells. These effects will be correlated with the mode of action of the antibiotic.