Rabbit antiserum was prepared against purified normal human liver phenylalanine hydroxylase. This was used to test for cross-reacting material in crude extracts of livers from patients with classical phenylketonuria (PKU); the samples from patients were two livers obtained at autopsy and a needle biopsy core. None of these enzymically inactive livers contained detectable cross-reacting material capable of neutralizing antibody activity in double immunodiffusion and/or enzyme inhibition experiments. In an earlier study, no phenylalanine hydroxylase was found in the two autopsy PKU livers by an affinity chromatography method or by the use of a specific antiserum raised against purified monkey liver phenylalanine hydroxylase (Choo et al., 1979a). This evidence suggests that mutations in th PKU patients studied may be regulatory in nature.