The number of colony-forming (clonogenic) cells in each of the crypts in mouse small intestine was deduced using a two-dose irradiation technique. The number was 7.5 ± 0.8 cells using two equal doses each less than 9 Gy and 38 ± 7 cells using 9 Gy or more per dose. The significant dose dependence could not be accounted for by considerations of intra- or intercrypt variability, or by the factor introduced to correct the sampling frequency for the influence of crypt size. The results suggest that more colony-forming cells may be recruited when the injury is more severe.