The influence of bcl-2 and bax expression on apoptotic cell death in mouse intestinal epithelia was assessed using homozygously null mice. Apoptosis was induced in vivo by the enterotoxin 5-fluorouracil (5FU) or by γ-irradiation and its cell positional incidence was assessed. 5FU and γ-radiation treated bax-null mice surprisingly showed no reductions in apoptotic yield in the small intestine or midcolon at 4.5 h at cell positions in which both agents had previously been shown to strongly induce p53 protein expression. The colonic epithelia of 5FU treated bcl-2-null mice showed elevated levels of apoptosis at 4.5 h: from 48 apoptotic events in wild-type mice to 273 in the nulls, scoring 200 half crypts. The increase occurred specifically in the cell positions considered to harbour colonic stem cells, at the base of crypts, where there is selective expression of bcl-2. There was a modest but significant increase in apoptosis in the small intestine of the bcl-2-null mice although the epithelia of wild-type mice here are not immunohistochemically positive for bcl-2 protein. These findings show that bcl-2 plays a key role in determining the sensitivity of colonic stem cells to damage-induced death but that bax is not responsible for the p53-dependent induction of apoptosis in this context.