The 38C13 murine lymphoma model was used to examine the role T cell activation plays in anti-CD3-based bispecific antibody therapy by comparing two bispecific antibody preparations that vary in their ability to activate T cells. Prior studies demonstrated that lower dose bispecific IgG (BsIgG) induced nonspecific T cell activation in vitro and in vivo, whereas bispecific F(ab')2 [bsF(ab')2] did not. BsIgG was more effective than bsF(ab')2 at prolonging survival of tumor-bearing mice. BsF(ab')2 was effective at prolonging survival when used along with IL-2. When used at a lower dose, the antitumor effect of both preparations was specific in that it was limited to cells bearing the target antigen. In contrast, larger doses of bsIgG, but not bsF(ab')2, resulted in regression of antigen-negative tumor, and so had therapeutic effects that were nonspecific. Serum from mice treated with larger dose bsIgG contained IFN-gamma that inhibited the growth of tumor cells in vitro. We conclude that two distinct mechanisms of action are playing a role in the observed antitumor effects. BsF(ab')2 and lower dose bsIgG inhibit tumor growth by retargeting T cells. Higher dose bsIgG also results in nonspecific T cell activation and systemic cytokine production that induces tumor regression.