We report here the identification of a ligand-receptor system that, upon engagement, leads to the establishment of an antiviral state. Three closely positioned genes on human chromosome 19 encode distinct but paralogous proteins, which we designate interferon-λ1 (IFN-λ1), IFN-λ2 and IFN-λ3 (tentatively designated as IL-29, IL-28A and IL-28B, respectively, by HUGO). The expression of IFN-λ mRNAs was inducible by viral infection in several cell lines. We identified a distinct receptor complex that is utilized by all three IFN-λ proteins for signaling and is composed of two subunits, a receptor designated CRF2-12 (also designated as IFN-λR1) and a second subunit, CRF2-4 (also known as IL-10R2). Both receptor chains are constitutively expressed on a wide variety of human cell lines and tissues and signal through the Jak-STAT (Janus kinases–signal transducers and activators of transcription) pathway. This receptor-ligand system may contribute to antiviral or other defenses by a mechanism similar to, but independent of, type I IFNs.