IL-15 administration does not induce GVHD in recipients of an allo TCD-BMT. Lethally irradiated (1300 cGy) CBA mice (A-G) received transplants of B10.BR TCD-BM (5 × 10⁶) or lethally irradiated (900 cGy) BALB/c mice (H) received transplants of B6 TCD-BM (5 × 10⁶) with or without T-cells. PBS (control) or 2.5 μg/d IL-15 was administered by intraperitoneal injection from days 0 to 10 (A-D, H), 10 to 20 (E, F) and 21 to 27 (G) after allogeneic BMT. T-cell dose was 0.25 × 10⁶ for panels C, E, and G and 0.5 × 10⁶ for panels D, F, and H. Open diamonds represent the PBS (control) groups, and filled diamonds represent the IL-15–treated groups in all GVHD experiments (C-H). Kaplan-Meier survival curves are shown (n = 8-16 mice each group).