The present invention relates generally to novel recombinant polypeptides having avian cytokine properties and to genetic sequences encoding same. More particularly, the present invention is directed to recombinant avian Type III interferon polypeptides, and genetic sequences encoding same, together with cellular expression systems and uses for same. Even more particularly, the present invention is directed to avian interferon-λ (IFN-λ) and functional derivatives, homologues and fragments thereof and to methods of use thereof. The molecules and cells of the present invention are useful in a wide range of applications including, but not limited to, providing a means for the treatment and prophylaxis of disease conditions, in particular avian disease conditions, or for use as an immune response modulator. Also provided are diagnostic means for screening for immune response and screening means for identifying modulators of IFN-λ protein or nucleic acid functionality.