SUMMARY:Faster growth of EscWchia coli strain B cells is observed in media which dontain factors previously found to increase the numbers killed by radiation. Incubation in the presence of metabolic inhibitors after irradiation allows many more irradiated cells to originate colonies. Holding the cells on complete media at low temperatures after irradiation causes more irradiated cells to die, an exception to the principle that greater survival of strain B is associated with slower growth. The difterence in the radiosensitivity of strain B and two resistant mutants, strains B/r and Bpr5, depends on differences in their utilization of organic nutrients, since: (i) all three strains have about the same sensitivity if they are plated on a minimal medium; (ii) if strains B and B/r are treated with chloramphenicol immediately after ultraviolet, the survival of strain B increases, and that of strain B/r decreases, both reaching the same level as if they had been plated immediately on minimal medium ; (iii) if irradiated cells of strsins B and B/r are incubated on nutrient medium for a short period before chloramphenicol treatment, the survival of strain B roughly equals that of the resistant mutants, while that of B/r is unaffected. When logarithmic phase broth-grown cells of all three strains are exposed to visible light after ultraviolet, only strain B is 'photoreactivated', and then only when viable counts on nutrient medium are compared, the counts on synthetic medium being unaffected by the visible light.

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