Mast cell–deficient mice have exacerbated GVHD. Recipient mice, which were either C57BL/6 (H-2^b) WT or mast cell–deficient C57BL/6-Kit^W-sh/Wsh mice, were lethally irradiated and transplanted with 5.0 × 10⁶ TCD-BM, followed by 2.0 × 10⁶ Tcon on day 4 (to induce GVHD) from FVB/N donors (H-2^q). Data were pooled from 2 independent experiments performed, each of which gave similar results (n = 13 mice per group except for C57BL/6-Kit^W-sh/Wsh with irradiation + TCD-BM, n = 10). Significantly better survival was observed in the control mice containing mast cells than in the mast cell–deficient C57BL/6-Kit^W-sh/Wsh mice (P < .0001).