We have demonstrated the in vitro IgE-mediated release of a prekallikrein activator from human lung. The lung prekallikrein activator was partially purified by sequential chromatography on sulfopropyl-Sephadex, DEAE-Sephacel, and Sepharose 6B. Purified human prekallikrein was converted to its active form (kallikrein) by the lung protease. The generated kallikrein was shown to be biologically active; that is, it generates bradykinin from purified human high-molecular weight kininogen and also cleaves benzoyl-propyl-phenyl-arginyl-p-nitroanilide, a known synthetic substrate of kallikrein. The lung prekallikrein activator differs from the known physiologic activators of prekallikrein (the activated forms of Hageman factor) with respect to: (a) size (it has a mol wt of approximately 175,000); (b) synthetic substrate specificity (D-propyl/phenyl/arginyl-p-nitroanilide is a substrate for the activated forms of Hageman factor, but not the lung protease); (c) antigenic specificity (an anti-Hageman factor immunoadsorbent column did not remove significant amounts of the lung protease, while it removed most of the activity of activated Hageman factor fragments); and (d) inhibition profile (the lung proteases was not inhibited by corn trypsin inhibitor). This prekallikrein activator provides a physiologic mechanism by which prekallikrein can be directly activated during IgE-mediated reactions of the lung. While the role of this lung prekallikrein activator in immediate hypersensitivity reactions and in other inflammatory processes is not clear, it does represent a first and important interface between IgE-mediated reactions and the Hageman factor-dependent pathways of the inflammatory response.