Tryptase is the most abundant protein constituent of the secretory granules of human mast cells, but little is known of the contribution of this serine proteinase in acute allergic reactions. We have purified tryptase from human lung tissue by immunoaffinity procedures, and have investigated its potential to provoke an inflammatory infiltrate in vivo. Within 6 h of injection into the skin of guinea pigs, the accumulation of large numbers of neutrophils and eosinophils was observed, and those eosinophils closest to the injection site were partially degranulated. Similarly, injection of tryptase into the peritoneum of mice, even in quantities as low as 5 ng, stimulated the ingress of neutrophils. The response was dose dependent at 3, 6, and 16 h, with increases in median numbers of up to 400-fold. At the later time points eosinophil numbers were increased by up to 10-fold, and there were elevations also in the numbers of lymphocytes and macrophages. In both models, the actions of tryptase appeared to be dependent on an intact catalytic site. Coinjection of heparin with tryptase had relatively little effect on tryptase-induced responses. On the other hand, although histamine did not itself stimulate cell accumulation, over a range of concentrations it altered the cellular composition of the infiltrate induced by tryptase. Addition of histamine to tryptase provoked selective increases in eosinophil numbers of up to fivefold in the mouse peritoneum. Tryptase may provide an important stimulus for granulocyte recruitment in allergic disease.