Two bispecific monoclonal antibodies (BsAb), differing in H chain isotype combination, were made for treatment of B-cell leukaemia/lymphoma; QAI-2, CD3-mouse-IgG1 x CD19-mouse-IgG2a and QAI-3, CD3-mouse-IgG1 x CD19-mouse-IgG2b. Both purified BsAb proved equally effective for their ability to target pre-activated T cells towards CD19 positive tumour cells. In T-cell proliferation assays, the capacity of Fc gamma RIa (CD64), Fc gamma RIIa-R131 and Fc gamma RIIa-H131 (CD32) transfected fibroblasts was tested to present the BsAb. The BsAb combining mouse (m) IgG1 and mIgG2a promoted T-cell activation in combination with the Fc gamma RIa transfectant; the mIgG1-mIgG2b BsAb was only marginally active. Both BsAb could not induce T-cell activation when presented by either of the Fc gamma RIIa transfectants. Similar results were obtained using PBMC cultures, containing Fc gamma RIa+/Fc gamma RIIa+ monocytes as accessory cells. The importance of Fc gamma R-dependent BsAb-mediated T-cell activation emerged from experiments with T cells and CD19 positive B-cell lines, showing that cross-linking via CD19+ target cells alone did not induce T-cell proliferation. Therefore, BsAb with functionally different Fc domains represent alternative strategies in BsAb therapy, the efficacy of which deserves to be compared in vivo.