The effect of methylglyoxal (MG), ascorbic acid and lactaldehyde has been tested on the in vitro respiration of Ehrlich ascites carcinoma (EAC) cells and several normal and malignant human tissues. Methylglyoxal inhibited the respiration of each type of malignant cell and tissue tested, but it had practically no inhibitory effect on the respiration of any of the normal cells and tissues. Ascorbic acid exhibited a synergistic effect with MG in inhibiting the respiration of all the neoplastic cells. In the presence of lactaldehyde, a catabolite of MG, the inhibitory effect of MG on the respiration of tumor cells was significantly reduced. Lactaldehyde can exert a similar protective effect on the loss of viability and transplantability of MG-treated EAC cells.