Tryptase, the most abundant protein product of human mast cells is emerging as an important mediator and target for therapeutic intervention in allergic disease. We have investigated the potential of tryptase and inhibitors of tryptase to modulate histamine release from human mast cells. Addition of purified human tryptase in concentrations ranging from 1 to 100 mU/ml stimulated a concentration-dependent release of histamine from cells dispersed from tonsil, although not from skin tissue. The reaction depended on an intact catalytic site being inhibited by heat inactivation of the enzyme, or by preincubating with the tryptase inhibitors APC366 or leupeptin or the tryptic substrate N-benzoyl-dl-arginine-p-nitroanilide (BAPNA). Tryptase-induced histamine release took approximately 6 min to reach completion, appeared to require exogenous calcium and magnesium, and on the basis of inhibition by antimycin A and 2-deoxy-d-glucose, seemed to be a noncytotoxic process. Preincubation of cells with tryptase at concentrations that were suboptimal for histamine release had little effect on their responsiveness to anti-immunoglobulin (Ig) E or to calcium ionophore A23187, but at higher concentrations their subsequent activation was inhibited. APC366 significantly inhibited histamine release induced by anti-IgE or calcium ionophore from both tonsil and skin cells, with up to 90% inhibition being observed at a concentration of 100 μM with skin. IgE-dependent histamine release was inhibited also by leupeptin, benzamidine and BAPNA. Tryptase may act as an amplification signal for mast cell activation, and this could account at least partly for the potent mast cell stabilizing properties of tryptase inhibitors.