Failure of tolerized CD4⁺ spleen cells to prevent the rejection of donor skin grafts by naive host-type CD4⁺ spleen cells. B6-SCID recipients were grafted with donor-specific (B10.A; panel A) and third-party (A.SW; panel B) tail skin grafts 8 days prior to the adoptive transfer of spleen cells. Day 0 is the day of adoptive transfer. All mice that received naive B6 splenocytes rejected B10.A skin within 28 days, regardless of whether spleen cells from chimeric mice were coadministered (recipients of the 3:1 ratio [n = 5] had a slight, but significant, prolongation compared with recipients of the 1:1 ratio [n = 4] and naive cells alone [n = 3]). Mice that received splenocytes only from chimeric mice accepted donor skin grafts for more than 60 days but rejected them by day 77 (n = 3). All recipients of spleen cells (regardless of origin) rejected third-party grafts within 19 days. SCID mice that received no spleen cells accepted both donor and third-party grafts indefinitely (n = 3). Chm and naive B6 indicate recipients of either spleen cells from chimeric or nonchimeric mice, respectively. 3:1 and 1:1 indicate the ratio of tolerant to naive CD4 cells given to recipients of both tolerized and naive splenocytes in combination.