To define further differences in antiviral activity as well as to identify candidate interferons for study in the prevention of rhinovirus colds, the antiviral activities of nine species of recombinant interferon alpha (IFN-alpha A, IFN-alpha B, IFN-alpha C, IFN-alpha D, IFN-alpha J, [Ser-116]IFN-alpha J1, IFN-alpha K, IFN-alpha J/C(Fnu4HI), and IFN-alpha A/D(BglII)) were evaluated against rhinovirus types 39 (RV 39) and 1A (RV 1A). WI-38 cells were exposed to various concentrations of each interferon and were then infected with RV 39, RV 1A, or VSV. Efficacy was determined by protection from cytopathic effect using a tetrazolium dye assay. The 50% inhibitory concentrations ranged from 4 +/- 3 pg/ml for IFN-alpha C to > 3000 pg/ml for IFN-alpha D against RV 39, and from 6 +/- 4 pg/ml for IFN-alpha J/C(Fnu4HI) to > 3000 pg/ml for IFN-alpha D against RV 1A. IFN-alpha J/C(Fnu4HI), [Ser-116]IFN-alpha J1, and IFN-alpha C were the most active of the interferons, and were all more active than IFN-alpha A, against RV 39, RV 1A, and VSV. These interferons warrant further study against rhinoviruses and other viruses.