Assessment of duodenogastric bile reflux has hitherto been unsatisfactory. An ambulatory system which utilizes the optical properties of bilirubin is examined. Test readings are correlated with the laboratory values for bilirubin in each of a number of physiological solutions. For dilutions of pure bile there was a linear correlation between absorbance and bilirubin concentration (r = 0.93, p < 0.001). In the more acidic environment of gastric juice there was also a linear correlation between absorbance and bilirubin concentration (r = 0.65, p < 0.001), but absorbance values were significantly higher than those of pure bile solutions (F ratio = 130, p < 0.0001). Normal gastric secretions and saliva give low absorbance values (0.02-0.04). Common foodstuffs such as soups, tea and coffee give higher readings and may interfere with bile reflux assessment in a clinical setting. Bilitec 2000 has potential for use as an ambulatory bile reflux monitoring system but consideration should be given to a standard diet or attention must focus on the fasting period.