One of the functions of surface CD38 is the induction of phosphorylation of discrete cytoplasmic substrates and mobilization of cytoplasmic calcium (Ca2+). The present work addresses the issue of whether the signaling mediated via CD38 operates through an independent pathway or, alternatively, is linked to the TCR/CD3 signaling machinery. We studied the signals elicited through CD38 by the specific agonistic IB4 monoclonal antibody (mAb) by monitoring the levels of cytoplasmic Ca2+ and the induced phenotypic and functional variations in T cell growth. IB4 mAb presented the unique ability to increase cytoplasmic Ca2+ levels, which correlated with the phosphorylation of the PLC-gamma1. These effects were blocked by phorbol 12-myristate 13-acetate (PMA) and were dependent on the presence of a functional TCR/CD3 surface complex, no effects being recorded on mutant Jurkat cells lacking part of the CD3 structures. CD38 signaling appeared to share with TCR/CD3 the ability to induce apoptotic cell death in Jurkat T cells, an event paralleled by specific up-regulation of the Fas molecule and inhibited by cyclosporin A. CD28, a costimulatory molecule, is synergized by increasing CD38-induced apoptotic cell death. The results indicate the existence of a strong functional interdependence between CD38 and TCR/CD3.