Dose-response curves for the reproductive capacity of crypt stem cells of murine colonic, jejunal and gastric mucosae exposed in situ to multifractionated .gamma. ray exposures were analyzed, and single-dose-survival curves of these cells were constructed. The single-dose-response curves bend downward over a dose range of approximately 200-1500 rad; cell death seems to be due to nonrepairable damage at doses less than 250 rad for colon and 220 rad for jejunum. There are 21, 110 and 140 stem cells per crypt of gastric, colonic and jejunal mucosa, respectively. Jejunal stem cells are the most radiosensitive, and gastric mucosal stem cells are the most resistant.