The induction of tolerance in a primed immune system is a major aim for therapy in autoimmunity and transplant rejection. In this paper, we investigate the action of the nondepleting anti-CD4 Ab, YTS 177. Although this Ab is nondepleting, we have demonstrated a direct action in vivo on activated effector cells. We show that the Ab inhibits transfer of insulin-dependent diabetes mellitus by the CD4+ Th1 clone BDC2. 5 to nonobese diabetic mice. Furthermore, we show that this Ab acts directly on diabetogenic effector cells because it prevented BDC2. 5-induced insulin-dependent diabetes mellitus in nonobese diabetic-scid recipients in the absence of other T cells. The Ab halts the diabetic process even when it is administered after the BDC2. 5 cells have infiltrated the pancreas and destruction of islets is already underway. This is accompanied by an immediate decrease in proinflammatory cytokine production with cessation of β cell destruction and disappearance of infiltrating cells from the pancreas, leaving any remaining β cells intact. These data suggest that Abs such as this may be effective not only because they induce regulatory T cells but also because they are able to directly prevent effector cell function.