Breastfeeding is required for inducing maximum NIMA effect to reduce GVHD. NIMA-exposed H-2^b offspring were produced by mating a B6 male and a B6D2F1 female, and were fed by either a B6D2F1 mother (NIMA [in utero + oral]) or a B6 foster mother (NIMA [in utero]). NIMA-exposed H-2^b offspring were produced using a B6D2F1 foster mother to nurse newborn B6 mice (NIMA [oral]). Lethally irradiated B6D2F1 mice received a transplant of 5 × 10⁶ TCD-BM from controls together with 2 × 10⁶ T cells from allogeneic or syngeneic donors. Survival (A) and clinical GVHD scores expressed as mean plus or minus SE (B) after BMT are shown. (A) The results are representative of 3 replicate experiments (n = 17-20/group). (B) Representative data from 1 of the experiments are shown (n = 6/group). *P < .05 compared with controls; **P < .05 compared with NIMA (in utero + oral).