Low-dose restriction has been demonstrated in the Ir-gene-controlled response to a variety of antigens. With regard to the murine antibody response to low (1 μg) doses of DNP-BGG in A1(OH) 3: strains A, CBA, and SJL are responders; DBA/1, C57BL/6, and BALB/c are nonresponders. In at least the last two strains, that nonresponsive state results from induction of tolerance. Although genetic factors play a role in determining susceptibility to induction of that tolerant state by the antigen in low doses, the type of adjuvant used with it is of fundamental importance. Thus, BALB/c mice, which are resistant to tolerance induction by ultracentrifuged BGG, are readily tolerized by administration of DNP-BGG in Al(OH) 3 or as an alum precipitate with DPT.