CHO cell lines that constitutively produce the murine interferon-alpha (IFN-alpha) subspecies alpha 4 and alpha 6 were constructed. The producer cell lines were protected against viral (vesicular stomatitis virus) infection by the IFN species secreted, but were resistant to the growth inhibitory activity of the IFN species. As compared with alpha 4, the alpha 6 protein displayed a high antiproliferative activity when added to normal CHO cells, which correlates completely with the high antiviral activity of alpha 6 on these cells. Three messenger ribonucleic acid (mRNA) species, which are normally induced in CHO cells by IFN treatment (1-8, 2-5A synthetase, and ISG 15) were constitutively present in CHO producer cell lines. The level of another mRNA (ISG 54), however, was very low in the producer cells as compared with its expression in short-term IFN-treated cells. These data indicate that 1-8, 2-5A synthetase and ISG 15 are not involved in the antigrowth activity of IFN in this system, but rather suggest a function of ISG 54 in this respect.