Monoclonal antibodies (MAb) specific for tumor-associated antigens (TAA) can induce an immunological cellular attack of tumor cells by a process termed antibody dependent cellular cytotoxicity (ADCC). Cytokines may augment ADCC by direct activation of immune cells or by enhancement of TAA on tumor cells. Thus, we investigated whether ADCC by MAb 17-1A and BR55-2, which recognize TAA on colorectal tumor cells, can be augmented by 3-day incubation with different concentrations of IL-2, IL-4, IL-6, IL-12, IFN-alpha, IFN-gamma, GM-CSF, M-CSF, and TNF-alpha. ADCC was assessed by a new flowcytometric cytotoxicity assay (Flieger et al. Immunol Methods 1995; 180:1-13) using PKH-2 labeled HT29 cells as targets and PKH-26 labeled peripheral blood mononuclear cells from three healthy volunteers as effector cells. We found three reaction patterns with the cytokines tested: (a) cytokines, which increase ADCC (IL-2, IL-12, IFN-alpha, and IFN-gamma, which represent Thl cytokines); (b) cytokines with no effect (GM-CSF, M-CSF, and TNF-alpha); and (c) cytokines, which decrease ADCC (IL-4 and IL-6, which represent Th2 cytokines). Then, we tested cytokines that increase ADCC in combination with the other cytokines. We found that the combinations IL-2/IFN-alpha, IL-2/IFN-gamma, IL-2/IL-12, and IL-12/IFN-alpha potentiated ADCC. By contrast, IL-4 reduced the IL-2, IL-12, and IFN-alpha-induced ADCC. Since the Thl response, cooperation of monocytes and CD4 cells is involved, we plan to elucidate by magnetic cell sorting (MACS) separation techniques, which cells are involved in cytokine-induced ADCC. Our results may be useful for finding combinations of cytokines and MAb for the locoregional treatment of colorectal cancer.