IL-7, a 25-kDa glycoprotein was cloned as a cytokine derived from bone marrow stromal cells, which induces proliferation of pre-B cells. IL-7 was found to be produced by the thymus, spleen, kidney and bone marrow stromal cells and induces proliferation of immature B and T cells. It has no effects on mature B cells but induce proliferation of mature T cells IL-7 R is expressed during the stages; B 220+ sIgM- of pre-B cells but not sIgM+ mature B cells, and also expressed CD4-8-, CD4+8- CD4-8+ T cells. It is shown that IL-7 can act only on these IL7R+ cells functionally in vitro and in vivo. We and other group demonstrated that IL-7, when used in combination with stromal cells or SCF can induce proliferation of pre-B cells. Recently we generated successfully SCF and IL-7 dependent pre-B cell lines, and now have used them for cell cycle analysis. To date, it was found that IL-7 is required for G1 to S transition. IL-7 also acts as a co-stimulus for the proliferation of antigen- or mitogen-activated T cells and can induce CTL response and LAK activity.