The number of clonogenic cells per intestinal crypt has been estimated in control and gamma-irradiated mice, from the response to a single or two test doses. Control unirradiated mice contained 43 ± 8 clonogenic cells per crypt, which was reduced to about 3 per crypt immediately after 8·0 Gy. After a mitotic delay which was approximately 18 h or 2·25 h/Gy the number of clonogenic cells per crypt increased exponentially with a doubling time of 21 ± 4 h to reach the control values by about the 4th day postirradiation. The growth curve was related to the changes in total cellularity and the cell production rate per crypt. Since both of these rise during the period of clonogenic regrowth some clonogenic cells must be diverted into the dividing transit cell population, so that the cell cycle time will be shorter than the doubling time given above.