Functional characteristics were established for a genetically engineered fusion protein between human IL2 and mouse/human chimeric mAb 225 directed against human epidermal growth factor receptor (EGFR), aberrantly expressed on human melanoma cells. The emphasis of these studies was on the mechanism(s) of action by which the ch225-IL2 fusion protein mediated cytotoxic killing of human melanoma cells by different human immune effector cells. Ch225-IL2 fusion protein bound to human EGFR with the high affinity of the parental antibody, and was as active as the equivalent amount of rhIL2. Ch225-IL2 enhanced cellular cytotoxicity mediated by freshly separated PBMC, isolated natural killer (NK) cells and activated T cells against melanoma cell lines. NK cells, which constitutively express both Fc gamma RIII and IL2R, interacted with ch225-IL2, mainly through Fc gamma RIII, while the involvement of IL2R was secondary. However, the effect of ch225-IL2 on activated T cells was most likely mediated through IL2R. These results suggest that the genetically engineered ch225-IL2 fusion protein may become a potent immunotherapeutic agent capable of stimulating various immune effector populations to effectively kill human melanoma cells.