SUMMARY Strains of Newcastle disease virus (NDV), which were good or poor inducers of interferon in chick cells, occurred in all virulence groups of this virus. Mesogenic and velogenic strains which killed the embryo rapidly were better interferon inducers in embryonated egg than were the lentogenic strains. Identical and low sensitivities to interferon were shown by an interferon-inducing vaccine strain and a non-inducing and highly virulent strain. Measurements made when various strains were used to infect either normal or interferon-pre-treated cells showed that the absence of interferon production was not due to the inhibitory effect of virus infection on cellular protein synthesis. It is suggested that infective NDV synthe- sizes two kinds of inhibitory protein during infection. One of these inhibits the synthesis of the interferon protein and in consequence the majority of strains do not induce interferon in chick cells. The second protein is induced mainly by mesogenic and velogenic strains and may be responsible for inhibiting cellular protein synthesis.