This present invention is directed to polypeptide and polynucleotide molecules that encode a four-helical bundle cytokine. The cytokine has been designated zsig81, and has restricted expression in primarily heart, lung and liver. zsig81 has been shown to stimulate proliferation of hematopoietic cells and will be useful expansion of these cells, as well as conditions associated with hematopoietic cells. The invention is directed to antibodies and methods of making zsig81 polypeptides, as well.