Lower monoamine oxidase (MAO) levels are reported in the blood platelets from chronic schizophrenics than in normal controls. Attempts to replicate these findings in other laboratories have been unsuccessful thus contradicting the suggestion that low MAO activity is a genetically determined biological factor in schizophrenia. We now present evidence to show that the endogenous amines present in platelets (serotonin) react non-enzymatically with the highly reactive aldehyde produced by MAO and thereby reduce the extractable radioactivity in the radiometric assays for MAO. Various biogenic amines such as serotonin, dopamine and m-tyramine were tested for their interference by adding them in nanomolar concentrations to incubation mixtures containing platelet samples or partially purified rat liver MAO and (14C-)p-tyramine or phenethylamine as substrates. The amines were added before, during and after incubation. In all three cases the apparent inhibition by each amine was the same, the percent inhibition depending on the structure of the amine.