Anesthesia and restraint without anesthesia during whole-body X-irradiation decrease the mortality from both the bone marrow and the intestinal syndromes (30- and 5-day mortality). The two types of immobilization decrease the radiosensitivity of the hemopoietic stem cells, as shown by an increased survival of hemopoietic stem cells in the marrow of immobilized mice. The hypoxic cell radiosensitizer Ro-07-0582 reversed the radioprotective effect during restraint without anesthesia, but not during pentobarbital anesthesia. This indicates that hypoxia of the femur bone marrow cannot explain the decreased radiosensitivity of the stem cells during pentobarbital anesthesia. Pentobarbital was also shown to inhibit the recruitment of resting femur bone marrow stem cells (${\rm G}_{0}\text{-phase}$ cells) into cycle following a sublethal dose of X rays. The relevance of these observations is discussed.

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