As Y. Okamoto 3 found, tumor cells can live without oxygen in serum containing glucose. Pieces of Flexner carcinoma after 24 hours deprivation of oxygen, Jensen sarcoma after 72 hours, could be transplanted with normal results. Hence tumor ceils are able to exist a certain time exclusively at the expense of fermentation. On the other hand, respiration alone is sufficient, without fermentation, to maintain the life of the tumor cell. We dialyzed rat serum in Ringer solution until it was free from glucose, and then kept pieces of tumor in this serum, passing oxygen through. Such pieces kept for some time in glucose-free serum and then transferred to serum containing glucose, showed normal respiration and fermentation, proving that most of the tumor cells were uninjured. Interfering with one of the two reactions furnishing energy to tumor ceils is not enough to kill them. It is necessary to stop both respiration and fermentation, if the cells are to be killed for want of energy. We achieved this by keeping pieces of tumor at body temperature in serum free from glucose and oxygen for various lengths of time. After a period of hours, the normal living conditions were restored by adding glucose and oxygen, and metabolism was measured. It was found that 4 hours interference with respiration and fermentation sufficed to kill off most of the cells, and to stop metabolism. We obtained similar results when we caused energy-want by killing tumor animals and allowing the tumors to remain in the bodies