Alterations in the negative charge of isolated nuclei after X-irradiation were investigated by electrophoresis in three different radiosensitive cell lines. The electrophoretic mobility decreased with time after irradiation and reached a minimum 4 hr after exposure. The relationship between the decrease in mobility measured 4 hr after irradiation and the radiation dose was linear and independent of cell lines utilized. In Burkitt lymphoma cells P3HR-1 (n = 1, $D_{0}=67$ R) the decrease in mobility was irreversible: The surving fraction of cells (colony-forming ability) and the fraction of electrophoretically intact nuclei 4 hr after exposure showed good correlation. In mouse melanoma cells B16-C2W (n = 20, $D_{0}=160$ R), however, the nuclear charge recovered almost completely by 24 hr after irradiation with doses up to 500 R, and good correlation was indicated between the surviving fraction (colony-forming ability) and the fractions of "electrophoretically intact" nuclei isolated 24 hr after exposure for five dose-points.

Rapid cell death of mouse intestinal crypt cells was investigated after 1000 rads of x-irradiation by counting pyknotic cells in tissue sections. The number of normal crypt cells decreased with the appearance of pyknotic cells after irradiation. Pyknotic cells appeared within 30 min and the number of pyknotic cells reached a maximum 4 hr after irradiation. From the disappearance rate of the cells the mean life span of pyknotic cells was estimated to be 2 hr or less. Pyknotic cells observed within 4 hr after irradiation were produced without cell division, and this interphase death by pyknosis occurred in all phases of cell cycle. The tendency for the cells to develop into pyknotic cells was high in G2 phase, intermediate in S phase, low in G1 phase. The increase in mitotic index and the decrease in pyknotic cells from the second to the fifth day after irradiation were inversely related. After the disappearance of pyknotic cells, rapid recovery of the cell population took place with logarithmic cell proliferation and shortening of the cell cycle. Recovery to the normal cell population level was attained within 5 days after irradiation.

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.