Mutations of the APC gene are thought to be early events in the process of colorectal carcinogenesis. Although the complete function(s) of the APC gene product is not known, it has been shown that the APC protein interacts with beta-catenin in a multi-protein complex to regulate the level of expression of beta-catenin. Loss of normal APC protein function can lead to an accumulation of beta-catenin in the cytosol and the nucleus. Immunohistochemical methods were used to determine the relationship between APC and beta-catenin protein expression in human colonic tissues (150 normal, 9 hyperplastic, 58 adenomas and 83 carcinomas) and 12 paired samples of normal and cancer tissue in mouse colon. In all samples of normal human and mouse colonic mucosa and in human hyperplastic polyps both APC and beta-catenin immunoreactivity were present in colonocytes. APC expression was cytoplasmic, with maximal immunoreactivity in the goblet cells. beta-Catenin expression was predominantly localized to the plasma membrane, with no nuclear immunoreactivity. APC immunoreactivity was absent in all of the mouse adenocarcinomas and 83% of the human colon cancers. All of the human and mouse carcinomas had nuclear and cytoplasmic beta-catenin expression. In contrast, only 29% of the 58 colonic adenomas were completely negative for APC immunoreactivity. Regardless of the presence or absence of APC, all of the adenomas had cytoplasmic and nuclear beta-catenin immunoreactivity. Many colonic adenomas retain expression of full-length APC protein whereas it is usually lost in colorectal cancers. Regardless of the status of APC protein expression, beta-catenin protein was found in the cytoplasm and nucleus of all neoplastic colonic mucosa. The dissociation between loss of expression of APC and accumulation of beta-catenin in the nucleus suggests that inactivation of both alleles of the APC gene may not be required for beta-catenin nuclear accumulation in colonic adenomas.