B CELLS ARE the only eukaryotic cells that are able to produce antibody molecules. Their growth and differentiation into antibody producing cells require the presence of T cells and macrophages; the function of these cells was found to be replaced by soluble factors. 3 In the early i980s it was shown that at least two different kinds of factors were required in the regulation of B cell response, one for growth of activated B cells, B-cell growth factor (BCGF), and the other for antibody induction in B cells, B-cell differentiation factor (BCDF). 6 Since then, a variety of factors regulating the B cell response have been reported in the human and murine systems. Finally, in 1986 the cDNAs for three B-cell stimulatory factors have been cloned; interleukin-4(IL-4)(BCGF1/BSF1) for the early activation of resting B cells, 7’8 IL-5 (BCGFII) for the growth ofactivated B cells, 9 and IL-6 (BCDF/BSF2) for the final differentiation of B cells into antibody producing cells’#{176}(Fig 1). Human IL-6 (BSF2) was originally identified as a factor in the culture supernatants of mitogen or antigen-stimulated peripheral mononuclear cells, which induced immunoglobulin production in Epstein Barr virus (EBV) transformed B-cell lines or in Staphylococcus aureus Cowan 1 (SAC) stimulated normal B cells. t1 This molecule was found to be separable from other factors, such as IL-2 and BCGFs, and the establishment of human T cell hybridoma generating BSF2 activity confirmed that this is a distinct molecule from other cytokines. 6 BSF2 was purified to homogeneity from the culture supernatant of a human T-cell leukemia virus type 1 (HTLV- 1) transformed T-cell line and its partial