Histaminase (EC-188.8.131.52), one of the two catabolic enzymes for histamine, is contained in human granulocytes. Opsonized zymosan or the calcium ionophore A-23187 induce a dose-dependent release of histaminase from human granulocytes in vitro. Release is completed within 30 min, is temperature dependent, and requires divalent cations. Opsonized zymosan-induced histaminase release was maximal in the presence of both calcium and magnesium, whereas ionophore release was magnesium independent. The total cellular content of histaminase could be released by both opsonized zymosan and ionophore. In contrast, only 25% of the cellular beta glucuronidase, a lysosomal enzyme, was released after maximal stimulation with opsonized zymosan; there was minimal release of beta glucuronidase with ionophore. Zymosan- and ionophore-induced histaminase release was inhibited by agents that are presumed to interfere with cell metabolism and disrupt microtubules. Human granulocytes therefore may modulate the effect of histamine by releasing histaminase at a site of inflammation. Studies of granulocyte histaminase release in vitro may also provide a new model to explore granulocyte function and secretion.