SUMMARYInterferon produced by mouse L-929 cells by incubation with poly(rI), poty(rC) is known to be composed of a mixture of MuIFN-c~ and MuIFN-fl. The c~ component was separated from the fl species by affinity chromatography over a monoclonal antiMuIFN-fl agarose column and partially purified by gel filtration. MuIFN-c~, prepared by this method was separated into at least five subspecies by chromatofocusing. The approximate pI values of these components are >~ 7.5, 6.5, 6.2, 5-9 and 5.6, respectively. Component 3 (pI 6.2) was the most prominent subspecies present in our MulFN-~ preparations, representing 40 to 50 ~o of the total antiviral activity. Component 1 (pI /> 7.5) which accounted for about 5~ of the antiviral activity on mouse cells, differed in some properties from the other interferon subspecies. It showed a relatively high antiviral activity on heterologous cells and it was eluted from a Sephadex column after the other c¢ subspecies. Furthermore, it showed a diminished binding to heparin as compared to the other MulFN-c¢ subspecies, indicating a lower affinity for polynucleotides.