Although uric acid is known to react with many reactive oxygen species, its specific oxidation products have not been fully characterized. We now report that 5-N-carboxyimino-6-N-chloroaminopyrimidine-2,4(3H)-dione (CCPD) is a hypochlorite (ClO-)-specific oxidation product of uric acid. The yield of CCPD was 40-70% regardless of the rate of mixing of ClO- with uric acid. A previously reported product, allantoin (AL), was a minor product. Its yield (0-20%) decreased with decreasing rate of mixing of ClO- with uric acid, indicating that allantoin is less important in vivo. Kinetic studies revealed that the formation of CCPD required two molecules of ClO- per uric acid reacted. The identity of CCPD was determined from its molecular formula (C5H3ClN4O4) measured by LC/time-of-flight mass spectrometry and a plausible reaction mechanism. This assumption was verified by the fact that all mass fragments (m/z -173, -138, -113, and -110) fit with the chemical structure of CCPD and its tautomers. Isolated CCPD was stable at pH 6.0-8.0 at 37°C for at least 6 h. The above results and the fact that uric acid is widely distributed in the human body at relatively high concentrations indicate that CCPD is a good marker of ClO- generation in vivo.