The presence of several covalent DNA adducts in the human colon was demonstrated by 32P-postlabeling in a previous study. We demonstrated that DNA of all the colonic mucosa tested were selectively adducted by a single genotoxic agent and this modification was completely absent in the DNA of muscular layers. In this study, the DNA adducts of the small intestine are compared with those of the colon to understand the role of mucosa-specific DNA adduct (MSA) in intestinal carcinogenesis. The mucosal DNA of the small intestine from 19 adults undergoing surgery due to gastric carcinoma (seven cases), pancreatic carcinoma (four cases), colon carcinoma (four cases), small intestinal tumor (two cases), intestinal trauma (one case) and ectopic pancreas (one cases) were analyzed. The mucosa-adjacent muscular layer DNA of the corresponding samples was examined as a control. Several common DNA adducts were observed in both mucosal and muscular layers of all the adults. Besides these common background DNA adducts, two MSAs (Si1, Si2) were detected in most of the adults as in colon cases. Si1 was present in all adults examined (19/19 cases) at a level of 0.04-0.22 adducts/10(8) nucleotides (average 0.09) and Si2 was found in 13/19 patients at a level of 0.03-0.08 adducts/10(8) nucleotides (average 0.05). Si2 was the same adduct detected in the adult colonic mucosa. However, they were absent in the adjacent muscular layer as well as in the neonatal intestine tested as a control. The total level of the mucosa-specific DNA adducts of the small intestine was approximately 28-fold lower than that of the colon. Considering that the incidence of cancer in the small intestine is rather lower than that in the colon, these results may be relevant to the development of human intestinal cancer.