Micrococcus radiodurans, irradiated aerobically in phosphate buffer, exhibits a phasic response to x-irradiation during its growth cycle. The culture progresses from a relatively sensitive state during the exponential portion of the growth cycle to a highly resistant state during the stationary phase. The shift in resistance is characterized by a marked extension of the shoulder of the survival curve and a threefold increase in the${\rm LD}_{99}$ from 250 kR to over 700 kR. The resistant state appears to be a metastable condition, readily induced in exponential-phase cultures and easily reversed in stationary-phase cultures. Similar variations are not elicited in response to ultraviolet irradiation. The phasic response does not appear to be the result of a low-molecular-weight extractable sulfhydryl radioprotective agent.

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