When early severe nausea and vomiting are combined with erythema and a depressed absolute lymphocyte count, one can consider it likely that the patient has been exposed to a high dose of ionizing radiation. The total white-blood-cell count with determination of absolute granulocyte and lymphocyte values is the most important diagnostic aid in this situation. Treatment is purely symptomatic. Hospitalization is advised for patients suspected of having received 150 r or more. Therapy is of the greatest value in the dosage range of 200 to 700 r, where hematopoietic failure and ensuing sepsis can be prevented by transfusions and antibiotics. If partial shielding has prevented injury to critical organs, the prognosis is better and the treatment can be modified accordingly.