Oral administration of Ag leads to systemic unresponsiveness (oral tolerance) to the fed Ag. Oral tolerance is mediated through active suppression by Th2 or TGF-β-secreting cells or clonal anergy/deletion, depending on the Ag dose used, with low dose favoring active suppression and high dose favoring anergy/deletion. The nature of APC and inductive events leading to the generation of oral tolerance have not been well defined. To determine the role of costimulatory molecules in the induction of oral tolerance, we have tested the effect of anti-B7. 1 or anti-B7. 2 mAb on the induction of tolerance by both high and low dose Ag feeding regimens. Our results show that the B7. 2 molecule is critical for the induction of low-dose oral tolerance. Injection of anti-B7. 2 but not anti-B7. 1 intact Ab or Fab fragments inhibited the oral tolerance induced by low-dose (0.5 mg) but not high-dose OVA (25 mg) feeding. In addition, anti-B7. 2, but not anti-B7. 1, inhibited secretion of TGF-β, one of the primary cytokines that mediates low-dose oral tolerance. Finally, in the in vivo model of experimental allergic encephalomyelitis, anti-B7. 2 mAb treatment abrogated protection offered against disease by low-dose myelin basic protein feeding, while anti-B7. 1 had no effect. Anti B7. 2 had no effect on disease suppression by high-dose oral Ag. These data demonstrate that B7. 2 costimulatory molecules play an essential role in the induction of low-dose oral tolerance.