The type I interferons (IFNs) are a family of homologous cytokines that compete for receptor binding. Past experiments with a cloned human IFN-α receptor component (designated herein as HuIFNAR-1) transfected into different cell backgrounds have given contradictory results in terms of binding and signalling after exposure of cells to different human type I IFNs. In order to investigate the binding specificity of human type I IFN subtypes to HuIFNAR-1, a cDNA encoding HuIFNAR-1 was transfected into simian COS cells. HuIFNAR-1 expression in COS cells, which was confirmed by Northern blot analysis, resulted in increased binding of 1 2 5 I-labelled HuIFN-α2 and -β. These data support the participation of this receptor component in ligand binding, probably in association with other receptor components.