Male mice of the BALB/c+ strain were exposed to X-rays at fractionation intervals of 7, 15, 30, and 60 days. One group received a mixture of radioprotectors, another AET (only 30 days fractionation), a third one served as control. The doses ranged, dependent on the treatment, from 300--1500 R. When survival was corrected for acute death, the control and AET treated animals died after an accumulated dose of about 2000 R whereas those treated with a mixture of radioprotectors died after about 4000 tr. Bone marrow failure and lung damage is the main cause of death within the initial 200 days after start of the exposure. At later times, fibrotic changes and in particular glomerulosclerosis are observed.