Monoamine oxidase content in extracts of human blood platelets was determined independently of MAO activity measurements with a recently developed monoclonal antibody against human platelet monoamine oxidase (MAO B) in a competitive radioimmunoassay. Administration of a single oral dose of the irreversible inhibitor tranylcypromine to normal human subjects resulted in a rapid and marked inhibition of platelet MAO catalytic activity within 1 day, followed by recovery of activity to normal levels within 2 weeks. In contrast, there was no change in the net concentration of MAO protein during this time. Since recovery of MAO B activity approximated the reported half-life of blood platelets, these results suggest that recovery of platelet MAO activity after tranylcypromine treatment is due to the replacement of old platelets by new ones which contain catalytically active MAO B.