Abstract
O-Hydroxyphenylacetic acid is present in the urine of phenylketonurics and, following phenylalanine dosage, in the urine of heterozygotes for phenylketonuria. It has now been found in the urine of normal individuals whose plasma phenylalanine has been raised during phenylalanine tolerance tests to a level comparable with that shown by the heterozygote. It must therefore be formed from phenylalanine by an enzyme other than one disturbed in phenylketonuria.