The sensitivity of the target cells responsible for the gastrointestinal syndrome in mice was deduced from the steepness of the dose-survival curve for mice assessed on Day 7 after irradiation. The D₀ value was 1.25 ± 0.22 Gy, virtually identical to the value of 1.23 ± 0.08 measured for microcolony-forming cells (clonogens) over about the same range of dose in concurrent experiments. The survival of clonogens was similar when assayed in mice surviving to Days 3, 4, or 5, but clonogenic sensitivity was lower when assessed on Day 7. This was shown at one dose to be due largely to a selection of mice with high colony counts with only a small contribution from crypt budding. The LD₅₀ for mice corresponded to a surviving fraction of crypts of about 0.35. An injection of 5 mg streptomycin sulphate ip daily for 5 days after irradiation increased the latent period by about 1 day, increased the LD₅₀ by about 1.4 Gy, but did not significantly change the survival of clonogens. These studies are the first to test and satisfy the interpretation of a dose-response curve for animal survival in terms of "target cell" survival, where measurements of both are made over a similar range of dose in concurrent experiments.