Blood cell production is regulated by a complex interacting network of stem and progenitor cells, from which all the blood forming elements are derived, and the effects of cytokines which can up- or down-modulate proliferation or self-renewal of stem and progenitor cells (1,2). This report reviews in brief recent information on the characteristics of human umbilical cord blood progenitor cells, the effects of the potent co-stimulating molecule, steel factor, and the myelosuppressive effects of macrophage inflammatory protein 1 alpha and other members of this latter group of molecules termed cytokines. In vitro as well as preclinical and clinical in vivo effects are covered.