We previously reported that T cells from aged mice were resistant to activation-induced cell death (AICD) in vitro. To determine whether the presence of AICD-resistant T cells is associated with defects in age-related peripheral clonal deletion in vivo, congenic male SCID mice were reconstituted with T cells from aged or young female Db/H-Y TCR (Tg71) transgenic mice. Compared with recipients of young cells, the recipients of T cells from aged mice exhibited a 3-fold increase in the percentage of autoreactive CD8+ H-Y antigen-reactive T cells as defined by the clonotypic antibody, M33. There were significantly increased sera levels of interferon-γ, a significantly decreased expression of FasL by M33+CD8+ T cells, and significantly decreased apoptosis by DNA fragmentation staining of the spleen of mice reconstituted with T cells from aged mice compared to those from young mice. By day 21, the recipients of T cells from aged mice but not young mice, exhibited infiltration of CD3+ cells into the non-lymphoid organs. These results indicate that there is defective peripheral deletion of the self-reactive T cells derived from aged female Tg71 mice, and that failure to delete these cells is associated with the defective T-cell clonal deletion in the recipient mice. © 2001 Elsevier Science Ireland Ltd.