The effect of low dose ionizing radiation from a variety of sources on the murine small intestinal and colonic pericryptal fibroblast sheath (PCFS) has been investigated. Ultrastructural and light microscopic changes seen within 3 hours after radiation resembles the mode of cell death known as apoptosis. The apoptotic index (AI) was shown to rise steadily for 4 hours after irradiation, and then to return to control levels by 10 hours. 137Cs gamma-irradiation over a range of doses from 0-803 cGy caused an increase in AI from control values of 1-2 per cent to a plateau level of approximately 8.5 per cent for doses above 40 cGy. Investigation of the response of the PCFS to doses of 137Cs and 60Co gamma-irradiation and X-irradiation below 40 cGy demonstrated a steady increase with dose in AI. The survival curve for the radiosensitive subpopulation susceptible to death through apoptosis as a consequence of exposure to non-acute doses of radiation has a D0 of 10.6 cGy and an extrapolation number of 2.3.