The response to radiation of murine bone marrow stem cells in different portions of the cell cycle has been obtained. Murine bone marrow stem cells were injected into suitable recipients and synchronized in vivo with hydroxyurea. At varying times thereafter, radiation was given, and surviving stem cells were assayed by the spleen colony technique. There was a 10-fold difference between the maximum survival at 9 hr and minimum survival at 10 hr. For determination of the position of the S (synthetic) phase of the cell cycle, another experiment was performed consisting of a second injection of hydroxyurea instead of radiation. A nearly inverse curve was obtained with a minimum survival at 8 hr. This allowed determination of the S phase of the cell cycle. It thus appears that murine bone marrow stem cells are most resistant in late S and are quite sensitive in the G2-M phase.

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This research was supported by USPHS Grant CA-10941 and by The William F. Milton Fund of Harvard University.

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