Radiobiological studies of the DBA mouse mammary carcinoma have shown that for single dose irradiation of 8 mm. diameter tumors the response is essentially the same for irradiations given under anoxia, air, O2 30 psi conditions. If, however, the irradiations are administered at 10 equal doses at 24 hour intervals, the ratio TCD50 anoxia/TCD50 O2 30 psi is 1.77, instead of approximately 1.08 for the single dose irradiation. This is further increased to approximately 2.7, when the radiation is administered at twice weekly instead of daily intervals. These results suggest that the effectiveness of high pressure oxygen in improving results of radiation therapy may be highly dependent on the dose fractionation schedule employed.