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Two interferon-mediated enzymes, a 2-5A synthetase and a kinase that phosphorylates a 67000 mol. wt. (p67K) protein were found at variable levels in different organs of mice. Among the different strains of mice included in this study, germ-free mice had the lowest levels of these enzymes. The levels of 2-5A synthetase and p67K kinase were enhanced significantly in all mice following treatment with mouse (α + β) interferon. Here, we show that the presence of 2-5A synthetase and p67K kinase in different organs of normal mice (untreated) was due, at least in part, to a constant production of interferon under different physiological conditions. Accordingly, injection of normal mice with anti-mouse interferon (α + β) globulin led to a significant decrease in the level of 2-5A synthetase and p67K kinase. In conventional mice (C3H/He), the level of both of these enzymes was higher in female than in male animals and was decreased with age or when such animals were reared isolated in a pathogen-free protected unit. The levels of 2-5A synthetase and p67K kinase were also decreased in normal mice following injection with a powerful antibiotic against a very wide spectrum of Grampositive and Gram-negative bacteria. These results suggest that the production of interferon was induced continuously in normal mice. Such induction was mediated by both internal and external agents.