SUMMARYThe coding part of a murine interferon alpha (MuIFN-c0 gene was cloned into an expression plasmid containing the simian virus 40 early promoter and the rabbit ~-globin polyadenylation signal. This construct was transfected into Chinese hamster ovary cells, together with a plasmid containing the Ecogpt gene as a selection marker. Resulting colonies were assayed for constitutive interferon production and analysed for integration of MulFN-ct genes. There was no obvious correlation between the number of genes integrated and the amount of interferon produced. The highest producer, designated CHO-pSVIOEF-3, contained four copies of the mouse gene and constitutively secreted up to 100000 International Units of interferon per ml per day. The MulFN-ct subspecies produced by this clone was characterized by analysis of its antiviral activity on heterologous cells, heparin-Sepharose affinity chromatography and chromatofocusing. The results obtained indicate that it is identical or closely related to a minor component present in conventional MulFN-~ preparations.