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 Cowan I (SAC) or anti-mu beads as co-mitogens. IL7 supported a dose-dependent proliferation of anti-mu-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%-80%) and large (50%-60%) B cells and then cultured with anti-mu beads, IL7 acted on both cell populations equally well. IL7 and BCGF (low molecular weight) were synergistic in their proliferative action on anti-mu-activated B cells in a 5-day culture. On the other hand, synergistic effect of IL7 on activated B cells was not evident in the presence of any other factor recombinant [(r)IL1 beta, rIL2, rIL3, rIL4, rIL5, rIL6, recombinant tumor necrosis factor-alpha, recombinant lymphotoxin, recombinant granulocyte-monocyte colony-stimulating factor and recombinant interferon-gamma] we tested. IL7 did not induce IgG secretion by activated B cells.