Increasingly sensitive assays are needed to understand and evaluate the effects of chemical exposures on individuals and populations. Several assays have been developed to measure the environmental dietary carcinogen, aflatoxin, and its metabolites in biological specimens. One, the 8,9-dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B1 nucleic acid adduct, has been shown to be both highly correlated with exposure and a strong predictor of carcinogenic outcome. Assays with increased sensitivity for this chemical adduct would be beneficial. Therefore, we have developed a hydrolysis reaction for the adduct found in urine, utilizing HCI acid and heat. Subsequently, quantification of the fluorescent metabolites produced can be obtained by either synchronous fluorescence spectrophotometry or high pressure liquid chromatography with fluorescence detection. The detection of lower levels of the adduct could prove helpful in the evaluation of risk in populations with lower exposures, such as those in chemoprotection trials or occupationally exposed groups.