Seven different human interferon (IFN) alpha subtypes were compared with regard to competitive inhibition of specific high-affinity binding of 125I-labeled IFN alpha-2 to human and bovine cells. All IFNs tested (alpha-1, -2, -4, -5, -6, -7, -8) competed for common binding sites. Marked differences in the binding affinities were observed between IFN alpha-1 and IFN alpha-2. The binding affinities of the various subtypes correlated with the respective specific biological activities determined in an antiviral assay. These observations suggest that at least some biological effects of IFNs are initiated by receptor activation and depend quantitatively on the degree of receptor saturation.