HeLa S-3 cells in nude mice and the parabasal and basal cells of nude mouse skins were exposed to graded dose rates (37 or 55 rad/hour and 200 rad/minute) and doses of radiation and analyzed in terms of their cell cycle distribution using flow microfluorimetry. Standard procedures were HeLa cells, and the parabasal and basal cells were harvested from the skin using dispase and trypsin. In the case of the tumor cells, in vivo, 37 rad/hour proved to be the most effective in producing G2/M accumulation, which is the sensitive phase of the cell cycle. The same dose rate proved to be the most efficient in producing G2/M accumulation in the parabasal and basal cells of the skin, but sensitivity to this effect of radiation was far less than in the tumor. This differential sensitivity of normal and malignant cells to chronic radiation-induced accumulation in a radiosensitive phase of the cells might play an important role in increasing the therapeutic ratio.