Survival in murine recipients with RENCA tumors undergoing syngeneic versus MHC-matched transplantation and in vitro cytotoxicity of NK cell subpopulations against Balb/C and C57BL/6 Con-A blasts and tumor cells. Survival of Balb/C mice challenged with RENCA tumor cells that underwent SCT with allogeneic B10.d2 (⋄) or syngeneic Balb/C (♦) splenocytes (15 × 10⁶) and bone marrow cells (8 × 10⁶) (top panel). Images show a representative picture of lung metastases in an allogeneic (day 53) and syngeneic (day 31) transplant recipient. Arrows show RENCA tumor foci in an allogeneic transplant recipient. Lungs in syngeneic recipients were replaced by tumor nodules. Significantly higher lung weights from increased tumor burden were observed at death in recipients of syngeneic (0.70 g ± 0.07 g) vs allogeneic (0.51 g ± 0.05 g; P < .01) transplants (A). NK cells isolated from CB6F1 mice by negative depletion magnetic beads (> 95% DX5⁺/CD3⁻) were flow sorted into Ly49G2⁺ and Ly49C⁺ subpopulations, then cultured for 4 days in IL-2–containing media and stained for purity by flow cytometry (B) and tested for lytic activity against Con-A blasts and RENCA (H-2^d) and B16 (H-2^b) tumor cells of Balb/C (H-2^d) and C57BL/6 (H-2^b) origin, respectively. Ly49C⁺/Ly49G2⁻ (○), Ly49G2⁺/Ly49C⁻ (•), and Ly49G2⁺/Ly49C⁻/Ly49D⁻ (▴) (C). NK cell subpopulations isolated from various strains of mice tested against Balb/C Con-A blasts and RENCA tumor cells (effector-target ratio [E/T] = 4:1) (D). Error bars depict SD of mean. One of between 3 and 5 representative experiments is shown. *P < .05 compared with opposing Ly49 NK group; unpaired t test. **The difference in survival between recipients of syngeneic and allogeneic transplantation is significant; P < .01, unpaired t test.