Wheat Germ Agglutinin (WGA) cytotoxicity has been studied using two human leukemia cell lines, Molt3 and K562, and human peripheral blood mononuclear cells (PBMC). In spite of similar binding at the cell surface, WGA was found to promote cell death to a different extent in Molt3, K562 and PBMC and to induce different death events leading to apoptosis in Molt3 and either apoptosis and necrosis in K562 cells and PBMC. In Molt3 but not in K562 cells, WGA cytotoxicity could be potentiated 66-200 fold by 50 nM monensin, a carboxylic ionophore that perturbs the intracellular trafficking of endocytosed molecules. Synergism between the cytotoxic activities of WGA and monensin was demonstrated in Molt3 cells by comparing non toxic, or slightly toxic, doses of WGA and monensin alone or in combination. These results show that the cytotoxic effect of WGA is dependent on internalisation events which may differ among the cell lines used. WGA and monensin can enter the human diet being a component of wheat germ and an antibiotic used for zootechnic reasons in the bioindustry, respectively. These data reveal the synergistic effect between two dietary molecules, otherwise per se toxic at much higher concentrations, with possible implications for human and animal health.