Monoamine oxidase B (MAO B) plays a pivotal role in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced Parkinsonism. An increased MAO B activity in platelets of patients with idiopathic Parkinson's disease (PD) is reported in this study. The possibility that high MAO B activity may represent a trait of vulnerability for PD by enhancing the neurotoxic effects of environmental compounds is discussed.