In this study we have examined whether differences between mouse strains in the induction of tolerance after feeding ovalbumin (OVA) are due to differences in intestinal processing of OVA or are determined by the systemic immune system. Compared with major histocompatibility complex (MHC)-congenic BALB/c mice, BALB/B mice develop much less tolerance of systemic delayed-type hypersensitivity (DTH) and humoral immunity after feeding OVA and this defect is also expressed partially in (BALB/B × BALB/c)F1 animals. Serum taken from either BALB/c or BALB/B mice fed OVA 1 h before produced significant suppression of systemic DTH responses in BALB/c, but not in BALB/B mice. Although OVA-fed BALB/B serum was slightly less tolerogenic than BALB/c serum, we conclude that the defective induction of oral tolerance in BALB/B mice is due primarily to a MHC-influenced defect with the immune system. These findings support the idea that clinical food-sensitive enteropathy reflects an immune response gene-controlled defect in tolerance to dietary proteins.