The 56-kilodalton (56K) protease isolated from a culture filtrate of Serratia marcescens caused vascular permeability enhancement followed by edema formation when injected into guinea pig peripheral corneas and subconjunctival space or skin. The character and the mechanism of permeability enhancement were analyzed in vivo. The enhancement was maximum at 5 to 10 min. The permeability reaction increased exponentially by the amount of enzyme used. The enhancement of permeability induced by the 56K protease was not affected by treatment with an antihistamine but was greatly augmented by simultaneous injection of a kinin potentiator, Glu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro-OH (SQ 20,881). Furthermore, the permeability activity of the protease, but not the amidolytic activity, was inhibited by soybean trypsin inhibitor, a well-known inhibitor of plasma kallikrein, as well as by corn trypsin inhibitor, the best inhibitor of activated Hageman factor. Results of these in vivo studies indicate that the permeability-enhancing reaction induced by the 56K protease is caused by activation of the Hageman factor-dependent pathway in the tissue. The permeability-increasing activity of the 56K protease was parallel with the enzyme activity. Serratial lipopolysaccharide did not produce a permeability enhancement reaction within 30 min when injected into guinea pig skin. These results are consistent with the results of recent in vitro experiments in which activation of the purified Hageman factor but not of prekallikrein by the 56K protease was elucidated (Matsumoto et al., J. Biochem. (Tokyo) 96:739-749, 1984). Thus, the molecular mechanism described above appears to be operative in the pathogenesis of corneal edema and chemosis, which is induced by S. marcescens, in addition to the direct tissue destruction by the protease.