Human tumor cells expressing ganglioside GD2 were lysed by various effector populations targeted with an anti-CD3-anti-GD2 bi-specific antibody (BAb CD3 x GD2). This antibodyheteroconjugate was prepared by chemically cross-linking the OKT-3 monoclonal antibody (MAb) reactive with CD3 antigen on T lymphocytes with the ganglioside MAb ME 361, which binds preferentially to the tumor-associated ganglioside GD2. The specificity of target-cell lysis by the cytotoxic T cells (CTL) was mediated by the specificity of the targeting antibody: GD2-negative cells were not lysed in the presence of the CD3 x GD2 BAb. A dose-dependent response was observed in a range of 10 to 10,000 ng/ml. In contrast, 2 other BAbs recognizing the tumor-associated antigens EGF-R and TKB-2 had greater potency to mediate tumor-cell lysis than the GD2 x CD3 BAb. Peripheral-blood cells (PBL) stimulated with OKT-3 MAb or with irradiated tumor cells in a mixed lymphocyte culture (MLTC) could be induced to lyse GD2-positive tumor cells in the presence of CD3 x GD2 BAb. The tumor-cell lysis could be mediated by autologous or allogeneic effector cells. NK cells had no influence on the BAb-induced cytotoxicity.