The recently identified CD27 ligand (L) is a type II transmembrane molecule with significant structural homology to tumor necrosis factor (TNF)-alpha, TNF-beta, lymphotoxin beta, CD40L, and CD30L. Using a CD27L specific mAb we examined the tissue distribution of the molecule, and found its expression to be restricted to B cells in occasional germinal centers, stromal cells in the thymic medulla, and scattered T cells in tonsils, skin and gut. As the limited expression of CD27L closely resembled the reported distribution of the activation antigen CD70, we tested whether CD70 represents the human CD27L. CD70 mAb were found to react with CD27L-expressing transfected mouse fibroblasts. Moreover a number of CD70 mAb could specifically interfere with the cellular binding of CD27L mAb. Thus, CD70 is identical to the human CD27L.