The michaelis constants (Vmax and Km) for platelet monoamine oxidase (MAO) with tyramine as substrate are found to be significantly lower in chronic schizophrenic patients than in normal controls. Furthermore, these kinetic parameters for the MAO of paranoid chronic schizophrenics are significantly lower than those for nonparanoid chronic schizophrenics. Paranoid chronic schizophrenia may be a separate biochemical disorder from other chronic schizophrenias.