MAb requirements at the time of BMT (second-stagetreatment). CBA mice were treated with non-depleting CD4 and CD8 mAbs alone 4 weeks before transplantation of 2 × 107 T cell depleted B10 BM. At the time of BMT the mice were treated with CD4 and CD8 mAb alone, or combined with CD40L mAb. Chimerism was only detected in the mice treated with both co-receptor and co-stimulation blockade at the time of BMT. Data is representative of 2 independent experiments.