Anti-CD3 MoAbs are widely used in T cell activation studies, and are effective in immunosuppressive therapy. We used a panel of mouse (m) anti-CD3 switch variant MoAbs of five different isotypes to study IL-6 release from accessory cells. Incubation of human (h) mononuclear cells with anti-CD3 MoAbs resulted in increased IL-6 levels with MoAbs of mIgG1 and mIgG2a isotypes, with no effect of mIgG2b or mIgA. This suggested involvement of IgG Fc receptors (Fc gamma R) in triggering IL-6 production. To evaluate the role of different Fc gamma R molecules individually we used a panel of hFc gamma R-transfected mouse fibroblasts, and Jurkat T cells as a model. IL-6 secretion by CD32 transfectants expressing the hFc gamma RIIa high-responder (HR) allelic form was triggered by mIgG1 anti-CD3 MoAb, with no effect of four other isotypes. None of the anti-CD3 MoAbs induced IL-6 secretion by CD32 transfectants expressing either a variant of this receptor, containing only a single intracellular amino acid (CT-), the hFc gamma RIIa low-responder (LR) allelic form, or hFc gamma RIIb1. hFc gamma RI (CD64) transfectants exhibited IL-6 production after incubation with mIgG2a anti-CD3 MoAb, and to a lesser extent with mIgG2b, and mIgG1 MoAb. Indirect involvement of T cells in triggering IL-6 secretion could be excluded by experiments in which transfectants were cultured with immobilized anti-CD3 MoAb. These data indicate that cross-linking of either hFc gamma RI, or hFc gamma RIIaHR by appropriate anti-CD3 MoAbs triggers IL-6 production of accessory cells, and not T cells. This may also take place in vivo during immunosuppressive therapy with anti-CD3 MoAbs, and related antibody-mediated immune responses.