The cell-cycle phase specific effect of an X-ray absorbed dose of 1.9 Gy on jejunal crypt cells of the mouse was studied in terms of mitotic delay and in terms of radiation induced cell death. A double labelling method with 3H-and 14C-thymidine was used in order to discriminate the cells in the different phases of the cell cycle. Cells irradiated during G2 phase are most radiosensitive, i.e. they exhibit the longest mitotic delay, followed in decreasing order by cells irradiated during early S and during the middle and end of S phase. Even some of the cells irradiated during G1 phase reach mitosis with a certain delay. A similar pattern of radiosensitivity is derived from the results on cell death. The death of cells irradiated at a higher phase age (G2 and S) is predominantly associated with the mitotic process, while the majority of cells irradiated during G1 and early S phase die out of interphase. A rough estimate has led to a mean life span of about 1 hr for radiation induced necrotic cells within the crypt.