• Plasma amine oxidase (PAO) activity was studied in 52 chronic schizophrenics, 130 first-degree relatives, and 36 normal control subjects. Enzyme activity was shown to be a heritable and stable characteristic. Age and sex effects were not present. Patients had lower PAO activity than did control subjects, although the difference fell short of statistical significance. Within families, reduced PAO activity was associated with schizophrenia spectrum disorders.