Three recombinant phages containing hamster interferon-alpha-encoding genes (Ha Ifa) were isolated from a Ha genomic library, using a murine (Mu) Ifa probe. The phage inserts contained overlapping genomic fragments which span a total length of approx. 30 kb, on which four Ha Ifa genes are localized. The Ifa gene cluster could be assigned to hamster chromosome 2q. The nt sequences of the four Ifa genes were determined. Two of the genes are functional (Ha Ifa-1 and Ifa-3) and two are pseudogenes (Ifa-ps2 and Ifa-ps4). Ha Ifa-1 and -3 were transiently expressed in COS cells and they gave rise to protein products (A1 and A3, respectively) with antiviral properties on hamster CHO cells. In addition, Ha A1 revealed high antiviral activity on murine L929 cells.