Treatment of mice with mAb to L3T4 profoundly depletes T helper cells. This treatment inhibits humoral and cellular immunity, retards autoimmunity, and permits the induction of Ag-specific tolerance. Treatment of BALB/c mice with F(ab')2 anti-L3T4 inhibits humoral immunity without depleting L3T4+ cells, which is evidence that mAb to L3T4 may inhibit T helper cell function in vivo. In this report, we demonstrate that F(ab')2 anti-L3T4 also permits the induction of immune tolerance in a manner that is independent of T cell depletion. C57BL/6 mice were treated with 1 mg of F(ab')2 anti-L3T4 every other day for 18 days and from the onset were challenged weekly with the immunogen 2C7, a rat mAb to chicken ovalbumin. These mice failed to respond to 2C7 not only during the treatment period but also for at least 5 mo thereafter. This immune tolerance was Ag-specific; the mice rapidly produced antibodies to subsequent challenge with another Ag, human gamma-globulin. Unlike intact anti-L3T4, which immediately depletes L3T4+ cells by greater than 90%, F(ab')2 anti-L3T4 did not initially deplete cells and caused only a partial reduction by the end of the 18-day treatment. This partial reduction of L3T4+ cells did not contribute to the induction of tolerance because mice that were first challenged with 2C7 3 days after stopping the F(ab')2 anti-L3T4 treatment, when L3T4+ cells were lowest, had a normal Ir to 2C7. These findings demonstrate that mAb to L3T4 permits induction of Ag-specific immune tolerance by a mechanism independent of its ability to deplete L3T4+ cells. They also show that F(ab')2 anti-L3T4 treatment does not impair humoral immunity when immunization is initiated after treatment is stopped. Because L3T4 is homologous to CD4 in humans, our findings suggest that F(ab')2 anti-CD4 may offer significant advantages over the use of intact anti-CD4 as an immunosuppressive agent in humans.