The ability of mouse mammary carcinoma cells, FM3A, to stain with trypan blue was evaluated as a function of time after x-irradiation with 1-30 krad. The number of unstainable cells did not change for the first 40 hr after 3-10 krad; thereafter the number of viable cells decreased exponentially with a mean lethal time of 33.6 hr. Control and irradiated cells were incubated at 37, 31, and 28°C, and the doubling time and mean lethal time showed the same temperature dependence, so that the activation energy for the expression of this type of radiation damage was the same as the activation energy for cell growth, suggesting that the expression of the damage requires cell metabolism.

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