When gamma-irradiated Escherichia coli B/r cells are reincubated after exposure, they separate into different populations during centrifugation on linear renografin gradients. The number of cells appearing in the most rapidly sedimenting band increases with increasing radiation dose. At the same dose, greater numbers of cells migrate to this zone when the irradiation is performed in the presence, as opposed to the absence, of oxygen. The band that sediments the greatest distance in the renografin gradient contains fewer viable cells than the upper band. After exposure to neutrons, electrons or uv radiation, the behavior of E. coli B/r on renografin gradients was similar to that observed after gamma-radiation. Heating the cells for 20 min or longer at 52°C also resulted in differential banding on renografin gradients. This suggests that metabolic activity of the bacteria during the actual heating period was of the nature required to "develop" damage leading to gradient separation of cells. In summary, we have developed a renografin gradient centrifugation technique which allows bacteria that have been exposed to various cytotoxic procedures to be separated into two major populations which differ in survival.

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