Studies on myeloid leukemic cells have identified other differentiation inducing proteins called D-factor and differentiation- inducing factor (reviewed in ref. 28). D-factor was identified as a protein that has also been called HILDA and leukemia-inhibitory factor, and differentiation-inducing factor was found to be a form of TNF. Another cytokine, IL-1, can also induce differentiation in some clones of myeloid leukemic cells, and this is mediated by induction of IL-6 (27). IL-6 and IL-i can induce viability and differentiation of normal myeloid precursors, and leukemia-inhibitory factor like IL-1 and IL-6 can synergize with IL-3 in normal myeloid colony formation. Three other cytokines, IL-11, oncostatin M, and ciliary neurotrophic factor, show some structural and functional similarities and use the same cell surface signal transducing protein, gpl30, that is used by IL-6 and leukemia-inhibitory factor (reviewed in ref. 37). These cytokines do not have CSF activity on normal hematopoietic cells, but IL-11 synergizes with IL-3 in normal hematopoiesis. There is another cytokine, IL-5, that induces differentiation to eosinophils (39). The IL-S receptor shares with IL-3 and GM-CSF receptors the i chain that is involved in signal transduction (reviewed in ref. 37).