Addition of exogenous IL-2 does not restore a proliferative response to donor antigens in early mixed chimeras. MLR assays were performed with splenocytes obtained 1 week after BMT against host-type (B6), donor (B10.A), and third-party (A.SW) stimulators. Chimeras were prepared with 3 Gy TBI, anti-CD8 and anti-CD154 mAbs, and BMCs, whereas conditioned controls received 3 Gy TBI, anti-CD8 and anti-CD154 mAbs, but no BMCs. nl B6 indicates untreated control B6 mice. Δ cpm = cpm against stimulator - cpm against “self” (recipient in the case of chimeras) stimulation. (A) No IL-2; (B) 2 U/mL IL-2; and (C) 5 U/mL IL-2.