Vaccinia virus penetrates, or is phagocytosed by, mouse leukocytes in vitro. A cytotoxic effect is observed, but no new infectious virus is produced. Vaccinia virus, as infectious particles, is eliminated from a culture of leukocytes at a more rapid rate than can be accounted for by thermal inactivation. Leukocytes infected with vaccinia virus produce a substance with the properties of interferon. The evidence presented suggests that leukocytes also produce interferon in vivo and that this interferon is related to the observed protective effect on the outcome of intracerebral vesicular stomatitis virus challenge in mice. It is postulated that leukocytes, in this manner, may make a positive contribution to the host's defense mechanism and to the process of recovery from viral infections.

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