Monoamine oxidase activity can be measured in human plasma. Though this enzyme is unlike intramitochondrial monoamine oxidases in platelet or brain, alterations of plasma monoamine oxidase activity have been reported in schizophrenia and depression. An electrophoretic study was undertaken to determine if genetic variants of the enzyme are involved in psychiatric illness. Plasma from 10 schizophrenic patients, 10 manic-depressive patients, 5 unipolar depressive patients and 8 normal controls was studied by polyacrilamide gel electrophoresis. No differences were found in the enzyme's electrophoretic pattern in the different diagnostic groups, and no reproducible individual differences were found.