Abstract: IT was found some years ago1 that dl-glyceric aldehyde inhibits glycolysis of tumour cells. Anærobic glycolysis is inhibited about 75 per cent by 10-3 M., whereas aerobic glycolysis is not affected by this concentration. Aerobically, it takes four times this amount to produce the same effect. Concentrations higher than 4 × 10-3 M. of the aldehyde are toxic.