Enzymic properties of partially purified monoamine oxidase (MAO) from human placenta were studied with tyramine, serotonin and benzylamine as substrates. The highest activity was obtained with serotonin and almost no activity was observed with benzylamine. These results are similar to those obtained with rat placental MAO, but different from those with rabbit placental MAO. The Km values for serotonin and tyramine were found to be 0.21 mM and 0.23 mM, respectively and the pH optimum was 8.1 with either substrate. The thermal inactivation curves of this enzyme with the two substrates were identical. The pI curves for inhibition of MAO activity by harmine, pargyline and iproniazid were similar and almost the same pI 50 values for the respective inhibitors were obtained with the two substrates. MAO in human placenta differs from that in other organs, such as liver, brain and plasma from the standpoint of the substrate specificity and the inhibitor sensitivity. The possibility that human placenta contains a single form of MAO is discussed on the basis of the present results.