Cultured Trichoplusia ni cells in exponential growth were administered X-ray doses of 10,000 R and then subcultured. The untreated cell population began exponential growth within a few hours after subculture, eventually reaching stationary growth phase 96 hr later at a cell density of <tex-math>$2.08\times 10^{6}$</tex-math> cells/ml, whereas the irradiated cell population did not change for 24 hr after irradiation and then began exponential growth at a rate similar to that of control cells, also reaching stationary phase at 96 hr, but at a cell density of <tex-math>$0.93\times 10^{6}$</tex-math> cells/ml, which is less than half the maximum density of controls. From 24 to 96 hr after treatment, the X-irradiated cells were characterized by an increased consumption of oxygen that was nearly twice the amount utilized by control cells. The pH of the cell growth medium increases over 96 hr from 6.3 to 6.6 for irradiated as well as for untreated cultures, but since the number of X-rayed cells is less than half the number of untreated cells, the pH increase, per cell, of medium from irradiated cultures is about twice that of medium from control cultures.

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