The sensitivity of fibroblastoid precursor cells in rat bone marrow to single and fractionated doses of γ rays delivered in vivo was measured. In vitro colonies were classified as being compact or diffuse, and the progenitor cells for both types were slowly cycling in vivo (survival levels after exposure to hydroxyurea were 90 ± 6% and 93 ± 11%, respectively). The progenitor cells forming diffuse colonies were more resistant ($D_{0}=1.39\ {\rm Gy}$) than those forming compact colonies ($D_{0}=0.76\ {\rm Gy}$). The fractionation sensitivities were characterized by an α/β ratio of 12.7 ± 5.5 Gy for diffuse colonies and 4.5 ± 3.0 Gy for compact colonies, respectively. The progenitor cells forming diffuse colonies may contribute more to long-term regeneration after high doses in vivo.

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