Recombinant human CD40 ligand (hCD40L) was expressed on the surface of CV1/EBNA cells and examined for its ability to induce proliferation and Ig secretion from human B cells in the presence or absence of soluble cytokines. hCD40L was directly mitogenic in a dose-dependent fashion for purified tonsil B cells with maximal proliferation occurring at days 5 to 7. Proliferation induced by CD40L was significantly enhanced in the presence of IL-2, IL-4, or IL-10 and strongly suppressed by transforming growth factor-beta. Although IL-5, TNF-alpha, and IFN-gamma had no stimulatory effect in the presence of hCD40L alone, if IL-4 was also present in cultures, these cytokines enhanced the proliferative response above that seen with IL-4 alone. Interestingly, in the absence of IL-4, IFN gamma had an inhibitory effect on hCD40L-induced proliferation. Although CD40L alone did not enhance Ig secretion, addition of IL-2 or IL-10 to the cultures significantly elevated the levels of IgM, IgG1, and IgA that were observed. Addition of IL-4 to the cultures did not enhance secretion of these isotypes but had a weak inhibitory effect. However, CD40L-mediated induction of IgG4 and IgE was dependent on the presence of IL-4. Of the cytokines examined, only IL-10 enhanced IgE secretion under these conditions. Although transforming growth factor-beta only partially inhibited secretion of IgM, IgG1, and IgA, it was strongly suppressive for IgG4 and IgE production. Our data demonstrate that proliferation and Ig secretion induced in the presence of CD40L can be modulated in a positive and negative fashion by soluble cytokines. IL-2 and IL-10 specifically enhance IgM, IgG1, and IgA production although IL-4, despite costimulating B cell proliferation, does not augment secretion of these isotypes but provided an essential cosignal with CD40L for the production of IgG4 and IgE.