The present invention relates to in vitro methods for stimulating, or increasing a population of, immune cells comprising using compositions which comprise a substantially pure polypeptide comprising a first amino acid sequence having at least about 70% homology to a sequence selected from SEQ. ID. NOs:7, 8, 9, and 11; or a biologically active fragment of any of these, wherein said polypeptide or fragment thereof can stimulate the proliferation and differentiation of granulocytes, monocytes, or dendritic cells; and to the use of these polypeptides in methods of treatment