The antiviral activity of human r-metIFN-con1 was compared with that of IFN-alpha 2b and IFN-beta on a number of human, other primate, rodent, feline, and canine cell lines. Although the specific activities of r-metIFN-con1 and IFN-alpha 2b differed 10-fold, the host range was very similar. The host range of IFN-beta differed from that of r-metIFN-con1 and IFN-alpha 2b in that Vero cells were 100-fold better protected by IFN-beta and MDBK protected at a 100-fold less efficiency. In general, there were only minor differences between the host ranges of the three interferons, human and primate cells being better protected than those of other species. However, the tissue of origin of the cell appears to be more important than the species of origin in defining host range [corrected].