The biotransformation activity of the mono-oxygenase enzyme, 7-ethoxycoumarin-O-deethylase (EOD), was investigated in the upper digestive tract of colon cancer patients, and compared with patients with and without neoplasia. The three groups studied comprised 23 control, 17 metastasized colon cancer and 16 metastasized breast cancer patients. EOD activity was determined by spectrofluorometry in duodenal biopsies obtained during gastroduodenoscopy. No correlation between the presence of colon or breast cancer and the level of EOD activity was observed. It was concluded that biotransformation enzymes can be easily determined in duodenal biopsies derived during. gastroduodenoscopy. The role of EOD in the biogenesis of colon cancer seems to be limited.