Signaling through the cell surface molecule, CD40, is known to play an important role in the proliferation and differentiation of B lymphocytes. Using the thymoma cell line EL4, we recently identified and cloned a cDNA encoding a murine ligand for the CD40 molecule (mCD40-L) and showed that it has biological activity in vitro. A cDNA encoding a human homologue of the mCD40-L was isolated using crosshybridization techniques from an activated peripheral blood T cell library. The predicted amino acid sequence indicates that this human ligand for CD40 (hCD40-L) is a 261 amino acid type II membrane protein that exhibits 78% amino acid identity with its murine counterpart. Northern blot and FACS analyses suggest that the hCD40-L is restricted in its expression to T lymphocytes, and that it is most abundant on the CD4+ T cell subpopulation. Cells transfected with hCD40-L caused the proliferation of human tonsil B cells in the absence of costimuli and, in the presence of interleukin 4, induced immunoglobulin E secretion from purified human B cells. A comparison of the efficacy of the hCD40-L and mCD40-L in these assays is presented.