Antigen-induced contractions of passively sensitized preparations of human bronchi and lung parenchymal strips were studied. Mepyramine did not affect the magnitude or time course of the contractile responses of either preparation. Indomethacin potentiated the responses of lung parenchymal strips, but inhibited those of bronchial preparations. Responses of both tissues were inhibited by the leukotriene antagonist FPL 55712, by the lipoxygenase inhibitor BW 755C, and by the anti-allergic compound disodium cromoglycate Disodium cromoglycate and BW 755C inhibited histamine release from lung parenchymal strips. The results indicate an important role for leukotrienes in allergic contraction of both large and small human airways, while histamine is of little importance. Bronchoconstrictor prostaglandins may also contribute to the response of large airways.