Apoptotic elimination of intestinal cells following irradiation has been studied in the small and large intestine of the rat, and correlated with the level of expression and localization of clusterin mRNA. Clusterin was moderately expressed in normal intestine where only small levels of apoptosis were found. After irradiation, however, there was a temporal correlation between an increased apoptotic index and increased clusterin expression. Localization of clusterin mRNA by in situ hybridization identified extensive labelling in the lower part (Paneth cell region) of small intestinal crypt, whereas epithelial cells in the large intestine were diffusely labelled. Clusterin expression was not localized over apoptotic cells and its role may be as a cell protection factor for surviving cells, as had been suggested by others. Clusterin may also be involved in remodelling of the intestinal crypt after radiation damage, a process that includes altering cell-to-cell contact, apoptosis, and sloughing of the dead cells from the intestinal villi. Our results do show a close temporal link between apoptosis and clusterin expression, and, as such, expression of the gene may be a useful indicator of presence of apoptosis in the irradiated intestine.