Population kinetic analysis was made for growth fraction and median cell cycle time over 9 days after irradiation of a mouse mammary tumor cell population (FM3A) treated with 500 and 700 R of ⁶⁰Co gamma-rays, where the growth fraction was estimated from labeling indices determined by the staggered addition method with ³H-thymidine and colchicine, and the median cell cycle time from the rate of c-mitosis accumulation by colchicine as well as the growth fraction. The analysis revealed a modest proliferation of the median cell cycle time of proliferating cells in the gamma-irradiated population. Irrespective of the doses of irradiation, the most remarkable reduction in the growth fraction was noted to occur on the third day of irradiation. The data obtained indicate that the diminution of the rate of increase in total cell number is derived primarily from decrement of the growth fraction and not largely attributable to prolonged cell cycle time, and the decline in the growth fraction is due to departure from the cell cycle and/or death of the cell.