Five populations of cells from a murine fibrosarcoma were separated in continuous gradients of Renografin in the density range of 1.05 to$1.18\ {\rm g}/{\rm cm}^{3}$. The clonogenicity of these cells was determined using a lung colony assay. The cell populations collected at densities between 1.05 and 1.09 were the more clonogenic. The radiation response of unseparated control cells and cells in each of the five separated populations was studied. The${\rm D}_{0}\text{'}{\rm s}$ ranged from 132 to 207 rad, the larger values being associated with the denser populations. The extrapolation numbers also varied; the higher values were found for the lighter populations. Only the population of cells banding at a density of 1.08 showed a response similar to that of the control, and this population was found to be the most clonogenic.

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