1. A premercapturic acid, i.e. a compound that yields a mercapturic acid when decomposed by acid, was isolated as a dicyclohexylammonium salt from the urine of rats and rabbits that had been dosed with bromobenzene. 2. Another premercapturic acid was isolated as its dicyclohexylammonium salt from the urine of rats that had been dosed with chlorobenzene. 3. When decomposed by acid, the premercapturic acid from the urine of animals dosed with bromobenzene gave p-bromophenylmercapturic acid, m-and p-bromophenol and NN′-diacetylcystine. 4. The premercapturic acid derived from chlorobenzene gave the corresponding chloro compounds together with NN′-diacetylcystine when decomposed by acid. 5. On the basis of these and other observations it is suggested that the premercapturic acid formed in the metabolism of bromobenzene is N-acetyl-S-(4-bromo-1,2-dihydro-2-hydroxyphenyl)-l-cysteine. 6. It is also suggested that the premercapturic acid derived from chlorobenzene has an analogous structure.
