Human skin fibroblast cells were treated with three types of human interferon (HuIFN), alpha, beta, and gamma separately, and in series of combinations at different concentrations. The IFN-induced enzyme, 2',5'-oligoadenylate (2-5A) synthetase, which is thought to be mediating the major part of the antiviral activity, was measured subsequent to 24 h treatment. The HuIFN-gamma potentiated the induction of 2-5A synthetase elicited by either HuIFN-alpha or HuIFN-beta, but the effect was seen only at low levels, i.e., 1-10 units. At higher levels, an antagonistic effect was seen. The individual subspecies of the HuIFN-alpha from native HuIFN-alpha were purified, separated, and analyzed for their ability to induce antiviral activity in human and bovine cells together with their capacity to induce 2-5A synthetase. Of the 12 species with molecular weights between 16,950 and 22,900 daltons, one species (MW 21,800) exerted peculiar properties in that it protected human cells better than bovine--at the same IFN level--and, it induced more 2-5A synthetase in human cells than in bovine cells.