The Fas/APO-1 (Fas) antigen is a cell surface protein that mediates apoptosis and belongs to the tumor necrosis factor receptor family. The lymphoproliferative (lpr) anomaly in mice results from a retroviral disruption within the fas gene. Mice that are homozygous for the lpr mutation accumulate large numbers of T lymphocytes and exhibit an autoimmune syndrome resembling systemic lupus erythematosus. A possible explanation for this process is that in the absence of Fas antigen, lpr T cells may be resistant to normal peripheral deletional signals. The bacterial superantigen staphylococcal enterotoxin B (SEB) rapidly induces anergy and deletion by apoptosis of reactive T lymphocytes in normal mice. Administration of SEB to adult lbr mice results in the delayed induction of both unresponsiveness and deletion of Vβ8+ lymph node cells. This is not due merely to an increased thymic output in lpr mice; the delayed induction of tolerance and elimination of reactive lpr T cells by superantigens are intrinsic properties of the cells. The progressive lymphadenopathy in lpr mice may reflect a process of lymphoaccumulation rather than lymphoproliferation. A delay in tolerance induction and elimination of selfreactive T cells could have profound influence on the autoimmune diathesis of lpr mice.