In recent years, several studies have confirmed the clonal elimination of thymocytes with receptors that recognize Ag and MHC molecules present on the membrane of thymic stromal cells, a process that may be relevant to the establishment of self-tolerance. In our work, we show that anti-CD3 treatment of single positive CD4+ or CD8+ human medullary thymocytes (obtained by anti-CD1a plus C) induces their apoptotic death. Some events commonly associated with the early steps of normal activation (IL-2R expression, increase in cytoplasmic Ca2+) are also induced after anti-CD3 treatment. Nevertheless, IL-2 is not secreted by these activated cells. The addition of exogenous IL-2 inhibits the apoptosis induced by anti-CD3. We suggest that the lack of secretion of IL-2 by medullary thymocytes may be a physiologic mechanism implicated in the process of negative selection that leads to tolerance.