Dogs were exposed to 250-kVp X-radiation while bearing a small external lead shield (weighing about 600 gm) around the limb epicondyle. Such shielding permitted survival after otherwise lethal doses up to 1000 R (a dose approximately 3 times the LD50. The protection was characterized by a critical recovery in the peripheral leukocyte counts usually observable by 12 days postirradiation. A single lead cuff was also effective against a 600 R dose of 1-mVp X-rays but not after 800 R. However, protection against mortality was evident at this radiation dose when double or triple layers of lead shield were employed. The theoretical basis for the findings is discussed in terms of critical functional repopulation of peripheral hemopoietic and lymphoid tissue sites via cellular seeding from the shielded marrow.