• Platelet monoamine oxidase (MAO) activity was investigated in a series of 60 patients with chronic schizophrenic illnesses free from neuroleptic medication, and 70 normal controls. No significant differences in platelet MAO activity or in substrate preference were detected between patients and controls, using tryptamine and tyramine as substrates. Platelet MAO activity did not distinguish patients with positive symptoms from those with negative symptoms alone and did not show any relationship with severity of disease or with a number of clinical features. These findings suggest that low platelet MAO activity cannot be regarded as a genetic marker for schizophrenia. Important determinants of platelet MAO activity yet to be discovered may have contributed to the discrepancies in recent observations of platelet MAO activity in schizophrenia.