Viral and bacterial infections may produce abnormalities in carbohydrate metabolism in normal subjects and profound changes in glucose homeostasis in insulin-dependent diabetics. Using an RIA with [125]porcine insulin, the effect of rat interferon (IFN) on the production/secretion of immunoreactive insulin (IRI) in cultured cells was investigated. We found that, after incubation of rat insulinoma cells with rat IFN, a decrease in the production of IRI was observed. The decrease correlated with the concentration of IFN and the length of the incubation. IFN also caused a decrease in the production/secretion of IRI from rat pancreatic beta-cells in primary culture. This IFN has all of the properties of IFN including species specificity, and the insulin inhibitory effect cannot be dissociated from the antiviral activity of IFN. These data suggest that one possible explanation for abnormalities in glucose metabolism associated with viral or bacterial infections is that virus-or bacteria-induced production of IFN may alter the concentration of insulin.