It is possible that the respiratory poisons, especially cyanides, give some cluesto the elucidation of the “oxygen effect”, which has been investigated in relation to the chemical protecting mechanism against X-ray irradiation. Some of the SH-compouncds, on the other hand, are known to show the protecting effect, of which, however, it is not known whether their reducing power is the only explanation for it.In these contexts, rates of the anaerobic glycolysis, which is known as one of the most sensitive metabolic activity to X-ray irradiation, of the Yoshida sarcoma cells were measured after their incubation under the following 3 sets of conditions, where the expelimental groups were irradiated, each control expeliment without irradiation being run at the same time. 1. In 0%, 20% and 100% oxygen, respectively. 2. with cystein, 6.7 × 10-2M. 3. with NaCN, 1.63 × 10-3M. The activities were measured after incubation by the routine Warburg's technique, with the following results : Anaerobic incubation, in itself, had no effect on the glycolytic activity. It lowered, however, the inhibiting action of X-ray by 15% compared with the aerobically incubated cells, thus showing a protecting effect. Incubation with 100% O2 had no effect. It seems that unphysiologically high O2 pressure obscures the oxygen effect.Cystein and NaCN were similar each other in stimulating the glycolytic activity. They showed, however, contrasting results when the cells were irradiated. The former increased X-ray inhibition by 39% (sensitivization), the cyanide, on the other hand, depressed it by 27% (Protection). The anoxic condition, which might be brought about by these compounds, must be different from the true anoxia in anaerobiosis. Their effects arc to be interpreted in the other terms of the radio-and biochemistry.