Antibody-dependent cellular cytotoxicity (ADCC) has been suggested to be an important defense mechanism against tumors. The effects of recombinant human macrophage colony-stimulating factor (rhM-CSF) on ADCC activity of human monocytes were investigated. Human peripheral monocytes were pre-incubated for 72 h with rhM-CSF at various concentrations (50, 100, 200, 400 U/ml) and then used as effector cells in a 24-h 111-Indium release assay. Human carcinoma cell lines LS-174T, CBS, and KLE were used as targets to react with anti-carcinoma monoclonal antibodies (mAbs: murine D612, murine CC49, and chimeric CC49). A significant increase in ADCC activity was observed after monocytes were incubated in 100-400 U/ml of human rhM-CSF. Variation in ADCC activity of monocytes among donors was observed. The enhancement of ADCC activity was blocked by the addition of a neutralizing antibody to rhM-CSF. Less D612 mAb was required for the rhM-CSF-treated monocytes to mediate an equivalent level of ADCC activity as compared to the untreated monocytes. Because of the low levels of rhM-CSF required in these studies to enhance ADCC, treatment of monocytes alone with comparable levels of rhM-CSF did not enhance antibody-independent cytotoxicity. Moreover, it is demonstrated here that recombinant human interleukin-4 (rhIL-4) and rhM-CSF can have a synergistic effect of monocyte-mediated ADCC on human tumor cells. These results thus indicate that rhM-CSF augments ADCC of human peripheral blood monocytes using mAbs to human carcinomas, suggesting a potential role for rhM-CSF in cancer immunotherapy.