Histaminase (EC 1.4.3.6) activity has been demonstrated in human eosinophils and neutrophils, but not in mononuclear cells, with the use of a new and specific thin-layer radiochromatographic enzyme assay. Leukocyte histaminase was physicochemically and functionally similar to histaminase isolated from human placenta and was principally localized to the 27,000-g granule-rich fraction of eosinophil and neutrophil homogenates. Histamine methyl transferase (EC 2.1.1.8), on the other hand, was detected in monocytes but not in granulocytes, eosinophils, lymphocytes, or platelets, and was localized solely to the 100,000-g cell sap supernatant fraction. These data suggest a role of human leukocytes in the catabolism of histamine and therefore in the modulation of histamine-mediated inflammatory reactions.