Mouse lymphocytic leukemia L 5178 Y in colonies showed the decreased radiosensitivity over that in free cell suspension. The decreased radiosensitivity is not sufficiently explained with the increased fraction of stationary phase cells, nor with cell-to-cell interaction, nor with protection by agar in the plating medium. The main cause is likely to be the depletion of oxygen in the multi-cellular system. The radiosensitivity of stationary phase cells in free cell suspension was slightly, but significantly lower than that of log phase cells.