The present invention relates to zcytor17lig polynucleotide, polypeptide and anti-zcytor17 antibody molecules. The zcytor17lig is a novel cytokine. The polypeptides may be used within methods for stimulating the immune system, and proliferation and/or development of hematopoietic cells in vitro and in vivo. The present invention also includes methods for producing the protein, uses therefor and antibodies thereto.