IL-10 production by IL-10/TGF-β–tolerized cells is not required for in vivo inhibition of GVHD mortality. Tolerized IL-10^–/–CD4⁺ T cells reduce mortality induced by naive cells. One hundred thousand naive B6 CD4⁺ T cells were injected with or without 105 tolerized IL-10^–/–B6 CD4⁺ T cells into sublethally irradiated bm12 recipients (n = 11 per group) (P < .001). Also shown is long-term survival of all recipients of 105 8-day MLR IL-10/TGF-β–cultured B6 IL-10^–/–CD4⁺ T cells (n = 10). Data represent 2 pooled experiments with similar results.