Requirement for donor Fas expression for maximal IL-12–induced GVHD protection. B6 mice received lethal irradiation followed by reconstitution with TCD B6 marrow, along with either (A) wild-type MRL or (B) with Fas-mutant LPR BMC and spleen cells (10⁷). Results of two experiments, both of which produced similar results, are combined (n = 11 to 12 mice per group). (—), GVHD controls; (---), IL-12–treated.