Ageing is associated with a progressive deterioration in the functions of many organs within the body. In tissue with high cell turnover, the maintenance of the stem cells is of particular importance. Any accumulation of damage in stem cells may affect their function and hence threaten the homeostasis and regenerative capacity of the tissue. The small intestine represents a good model for the study of stem cells because of its spatial and hierarchical organisation. We have examined the effect of age on stem cell regenerative capacity after irradiation, using the microcolony assay. Crypt survival levels, the growth rate of surviving crypts, and the number of cells able to repopulate a crypt have been investigated by irradiating groups of 67 month old and 28-30 month old ICRFa male mice. After high doses of irradiation, the surviving crypts in old mice were both smaller and fewer in number than in young mice. The growth rate of surviving crypts was determined by measuring the crypt area and the number of cells/crypt at various times after 14 Gy irradiation. There was a growth delay of between about one half and one day in the older mice. Surprisingly, the number of clonogenic cells per crypt was estimated to be greater in the older mice. These studies indicate important age-related alterations in the capacity to regenerate the crypts after radiation damage.