A dose of 900 rad gamma-rays to mice reduces the number of cryptogenic cells (those capable of crypt regeneration) per small intestinal crypt from about 80 to about 2 per surviving crypt. The surviving cells repopulate the crypt, the estimated cell-number doubling-time being about 24 hours, and on the fourth day after irradiation their numbers are still slightly below normal. The regeneration patterns for cryptogenic and proliferative cells differ significantly over the first 4 days after irradiation. The numbers of proliferative cells fall initially and remain low while the cryptogenic cells are increasing in number. The low levels persist for the first 1-5--2 days, after which there is a rapid rise in proliferative cells per crypt with a significant overshoot on the third day and a peak on the fourth day. The number of proliferative cells per crypt in control animals is about twice the number of cryptogenic cells. This ratio is on average 12 after irradiation (days 1--4). These data add support to the hypothesis that the small intestinal crypts contain a sub-population of stem cells that are distinct from the proliferative cells.