In cancer radiotherapy, an enormous radical dose may be necessary. However, it is sometimes limited by the tolerance of the surrounding normal tissues. Transient systemic hypoxia induced by breathing 10.5% oxygen gas during radiotherapy may protect the normal tissues. In the prophylactic irradiation to the whole abdomen and pelvis for the NHL or ovarian cancer, clinical radiation responses were distinctly mild and occurred later in the hypoxyradiotherapy group. Micronucleus frequency of the peripheral lymphocytes was taken as the criterion in estimating the radiation effect. A radioprotective factor (RPF) of 1.52 was obtained.