Protection against GVHD depends on IL-4 production by NKT cells. 1.0 × 10⁴ NKT cells (DX5⁺TCRβ⁺CD4⁺) were sorted from C57BL6 donors (H-2^b) and transferred into BALB/c mice (H-2^d) that had received T cell–depleted bone marrow and 5.0 × 10⁵ Tcons (to induce GVHD). The source of NKT cells was either WT or IL-4^−/− KO mice. Data indicated that the transfer of 1.0 × 10⁴ WT NKT cells resulted in significantly improved survival over the transfer of 1.0 × 10⁴ IL-4^−/− KO NKT cells (P = .02). Groups: irradiation control (no BMT; ■); TCD-BM only (●); 5 × 10⁵ Tcons (▾); Tcons + 1 × 10⁴ WT NKT (○); and Tcons + 1 × 10⁴ IL-4^−/− NKT (◇). Data in graph are pooled from 3 independent transplants (n = 15).