We cloned the human CD40 ligand (hCD40L) from a cDNA library constructed from an activated CD8+T cell clone. Two cDNAs representing a 2.1 and a 1.4 kb clone were detected. Both cDNA clones had identical open reading frames of 261 amino acids and differed only in the length of their 3′ untranslated ends, and probably represent the 2.1 and 1.4 kb mRNA species detected by Northern analysis in an activated CD4+ T cell clone. hCD40L transcripts could also be detected in CD4+ and CD8+ TCR αβ T cells, TCR γδ T cells, natural killer cells, monocytes, small intestine, and fetal thymocytes, but not in purified B cells, fetal liver, fetal bone marrow, brain, kidney, or heart. COS-7 cells transfected with hCD40L (COS-7/hCD40L) induced human B cell activation as Judged by the induction of homotypic aggregates of Epstein-Barr virus transformed and normal B cells. In addition, COS-7/hCD40L induced B cell proliferation, which was further enhanced by IL-4, or IL-13. IL-13, like IL-4, synergized with the mouse and hCD40L to induce IgM, total IgG, IgG4, and IgE, but not IgA, production by highly purified B cells. Anti-IL-4 antibodies inhibited IL-4 and COS-7/hCD40L induced Ig production by B cells, but had no effect on IL-13 and COS-7/hCD40L induced B cell differentiation, indicating that IL-13 and hCD40L induced Ig production, including isotype switching to IgE, independently of IL-4. hCD40L induced B cell differentiation was blocked by soluble CD40, confirming the requirement for specific engagement of CD40L. Collectively, these data indicate that CD40L and IL-13 expressed by human CD4+ T helper cells are important components of T-B cells interactions resulting in B cell proliferation, differentiation and IgE switching. However, the distribution of the hCD40L suggests a broader function of this molecule.