Autoimmune diseases can be characterized by increases in Th cell activities, suggesting that inhibition of Th cell function might ameliorate autoimmunity. We have recently reported that administration of nonmitogenic anti-CD3 mAb (nmCD3) to nonautoimmune mice can induce long-term Th cell hyporesponsiveness, reflected by reduced IL-2 secretion upon re-exposure to Ag. This study was designed to determine the effects of nmCD3 on autoimmunity by using the murine collagen-induced arthritis model. Treatment of DBA/1 mice with nmCD3 delayed the onset and reduced the severity of arthritis in mice immunized with type II collagen (CII). This effect was not caused by depletion of T cells or modulation of TCR. The observed inhibition of arthritis was not caused by decreased Ab production, as anti-CII titers were not affected. Rather, lymph node cells from CII-immunized mice treated with nmCD3 were hyporesponsive to in vitro stimulation with CII. This hyporesponsiveness was reflected by a marked decrease in secretion of IL-2 and IFN-gamma, but not of IL-4, which suggests that nmCD3 had its principal effect on Th1 cells. The hyporesponsiveness was not Ag-specific, because IL-2 and IFN-gamma production in response to a pan-T cell mitogen was also reduced. These results demonstrate that induction of Th1 cell hyporesponsiveness with nmCD3 can significantly alter the course of CIA and suggest that IL-2 and/or IFN-gamma play a crucial role in disease pathogenesis.