C57BL/6-Kit^W-sh/Wshmice that had received BMCMCs intraperitoneally have improved survival during GVHD induction. Recipient mice, which were C57BL/6J (H-2^b) WT, mast cell–deficient C57BL/6-Kit^W-sh/Wsh, or C57BL/6-Kit^W-sh/Wsh engrafted with WT or IL-10^−/− BMCMCs were given lethal irradiation 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 the 4 independent experiments performed, each of which gave similar results (n = 10 for mast cell–engrafted groups; n = 23 for all other groups). Significant improvement in survival was observed when C57BL/6-Kit^W-sh/Wsh recipients had been engrafted 8 weeks prior to transplant with WT BMCMCs (P < .01); however, significant improvement in survival was not observed if BMCMCs were derived from IL-10^−/− mice. Furthermore, engraftment of WT BMCMCs into C57BL/6-Kit^W-sh/Wsh mice produced a survival curve not significantly different than that of WT C57BL/6J recipients.