THE recessive mouse mutations lpr and gld create deficiencies in an interacting pair of cell surface molecules, CD95 (Fas/APO-1) and Fas-ligand (FasL), respectively(1-3), resulting in autoantibody production resembling human systemic lupus erythematosus(4). The mechanisms of self-tolerance affected by deficiency in either molecule are not established, but CD95 deficiency both in B cells and in CD4(+) T cells recognizing major histocompatibility complex (MHC) class II molecules is required for autoimmunity in lpr mice(5-8). Here we track the outcome of in vivo interactions between B cells and CD4(+) T cells that recognize a transgene-encoded autoantigen, hen egg lysozyme (HEL), using cells from mice transgenic for immunoglobulin and T-cell receptor (TCR) genes. B cells that had not previously encountered HEL autoantigen (naive cells) were triggered into proliferation and antibody-production upon interaction with antigen and HEL-specific CD4(+) T cells. By contrast, B cells that had been chronically exposed to HEL during their development and carried desensitized surface immunoglobulin (sIg) antigen receptors(9) (anergic cells) did not produce antibody but instead were eliminated in the presence of HEL-specific CD4(+) T cells. CD95-deficient anergic B cells, however, were not eliminated by CD4(+) T cells and were triggered to proliferate. These findings identify a novel regulatory step for eliminating autoreactive B cells that seems unique in its dependence on CD95.