Groups of 5 lethally irradiated (1,100 cGy) BALB/c recipients were transplanted with grafts containing 40 × 10⁶T-cell–depleted B6 or B6-Ly5.1 marrow cells alone (control) or with 1.0 × 10⁷ CD3 cells from perforin-deficient, Fas-ligand–defective donors added to the graft (mutant). Body weight was measured as a percentage of the pretransplant baseline, and the percentage of B220⁺, CD3⁻ cells in the blood lymphoid gate was measured by two-color flow cytometry among recipients surviving on the indicated day when the experiment was terminated. Shown are mean values ± 1 SEM for three consecutive experiments. In each experiment, the differences between groups were statistically significant. Experiments no. 1 and 3 were performed with mutant cells from mice, with perforin expression disrupted by an insertion at the exon 3 BstEII site. Experiment no. 2 was performed with mutant cells with perforin expression disrupted by an insertion at the exon 3Sph I site.