Novel polypeptide combinations, polynucleotides encoding the polypeptides, and related compositions and methods are disclosed for soluble zalpha11 receptors that may be used as novel cytokine antagonists, and within methods for detecting ligands that stimulate the proliferation and/or development of hematopoietic, lymphoid and myeloid cells in vitro and in vivo. Ligand-binding receptor polypeptides can also be used to block zalpha11 Ligand activity in vitro and in vivo, and may be used in conjunction with zalpha11 Ligand and other cytokines to selectively stimulate the immune system. The present invention also includes methods for producing the protein, uses therefor and antibodies thereto.