We investigated the capacity of human interleukin (IL) 7 to induce proliferation of B cells. Purified tonsillar B cells were cultured in the presence of IL7 with Staphylococcus aureus CowanI (SAC) or anti-p beads as co-mitogens. IL7 supported a dose-dependent proliferation of anti-p-activated B cells but did not significantly support proliferation of SAC-activated B cells. When B cells were separated on Percoll gradient into small (60% -SOYO) and large (50% -60%) B cells and then cultured with anti-p beads, IL7 acted on both cell populations equally well. IL7 and BCGF (low molecular weight) were synergistic in their proliferative action on anti-p-activated B cells in a 5-day culture. On the other hand, synergistic effect of IL 7 on activated B cells was not evident in the presence of any other factor recombinant [(r)ILlP, rIL2, rIL3, rIL4, rIL5, rIL6, recombinant tumor necrosis factor-a, recombinant lymphotoxin, recombinant granulocyte-monocyte colony-stimulating factor and recombinant interferon-y] we tested. IL7 did not induce IgG secretion by activated B cells. 1 Introduction B cell response to antigen or mitogen takes place in three sequential steps known as activation, proliferation and differentiation. Human B cell activation can be induced by Staphylococcus aureus Cowan strain I (SAC) or by anti-IgM antibody [l, 21. Following activation, human B cells can undergo proliferation and differentiation into antibody-secreting plasma cells under the influence of various lymphokines derived from T cells. Some of these lymphokines influence various stages of B cell development while others act at a particular stage. IL2 is capable of both proliferation and differentiation of B cells and does not activate resting B cells [3,4]. However, IL4 activates resting B cells, stimulates growth of activated B cells and plays some role in differentiation [5,6]. While IFN-y enhances B cell proliferation and Ig secretion in the early stage [7], IL3 and IL6 are effective at the later stage of differentiation [8-lo]. Two other growth factors, low-and high-mol. wt BCGF, have been shown to promote B cell proliferation [ll-131. Recently, a new lymphokine, IL7,was cloned [14]. IL7 was initially purified from the SN of cultured BM stromal cells and shown to act on pre-B and pre-Tcells in mice [15-171 and humans [14, 18, 191. Later, it was also found to act equally well on human [20,21] and murine [22, 231 mature T cells and was shown to synergize with various lymphokines in its action on murine thymocytes [24,25]. On the other hand, there was not any effect of IL7 on mature murine B cells even in the presence of anti-p beads [26]. Very little information is available on the action of IL7 on human B cells.Welch et al. [27] showed that human (hu)IL 7 [I 88151