Groups of mice were given 400 to 550 units of x-ray. Estrogen was given 1/3 of the mice at the time of irradiation. 1/3 of the mice had received estrogen 9 days before irradiation. At the end of 4 days the survival of the irradiated mice was 20.6%, of mice given estrogen and x-rays at the same time, 11.7% and estrogen 9 days before irradiation, 75%.