Murine monoclonal antibodies (MAB) of the IgG3 subclass generated to colorectal tumor-associated glycolipids were assessed for enhancement of antibody-dependent cell-mediated cytotoxicity (ADCC) by interleukin 2 (IL-2). Mononuclear cell preparations containing large granular lymphocyte effectors required only low doses of IL-2 (0.1-10 units/ml) and short exposure (3 h) for maximal enhancement of ADCC. Exposure of mononuclear cells for longer periods (1-6 days) to higher levels of IL-2 (100-1000 units/ml) resulted in the generation of lymphokine-activated killer N cells which were lytically active without antibody. A non-ADCC MAB of the IgG1 subclass showed no ADCC even after stimulation of effector cells with IL-2. Effector cells pretreated with IL-2 showed an enhanced rate of cytolysis of target cells in the presence of antibody. Pretreatment of effector cells with IgG3 MAB resulted in lower ADCC, but pretreatment of target cells or simultaneous addition of MAB and effectors to target cells gave higher levels. These results indicated that ADCC with murine IgG3 is enhanced by levels of IL-2 achievable in current patient trials. The combination of antibody and IL-2-boosted effector cells gives comparable levels of killing to lymphokine-activated killer cells but should not suffer from similar toxicity. The NR-Co-04 antibody in combination with lymphokine may prove effective in treating colon cancer, a disease for which both chemotherapeutic and current biological therapies suggest a need for improved forms of therapies.