Abstract. Intravascular injection of purified 70-kDa protease or GCAT-LPS from Aeromonas salmonicida, or both, invariably led to consumptive coagulopathy within 2h in Atlantic salmon, Salmo salar L. By entering the coagulation cascade at two different levels, the two enzymes work in concert in thrombus formation, the significance being that circulatory failure is probably the major cause of death in acute furunculosis. Only intravascular injection of GCAT-LPS led to consumptive coagulopathy in rainbow trout, Oncorhynchus mykiss (Walbaum), despite the fact that trout received only half the GCAT dose, and/or four times the protease activity administered to salmon. These results indicate that salmon is, by far, the most susceptible to protease and suggest that rainbow trout is the most susceptible to GCAT-LPS. The substrate profile of the purified protease gave supporting evidence that it works as activated coagulation factor X. The protease is inhibited in vitro by antithrombin and by α2-macroglobulin and both inhibitors are consumed in vivo in response to intravascular administration of the enzymes, thus showing a potential for AT and α2M to inhibit the protease also in vivo. Provided such plasma antiprotease activities are correlated with resistance to the disease, and the inhibitors show genetic variation they would be promising candidates for indirect selection and hence a means to prevent furunculosis independently of vaccines. Circulating inhibitors of GCAT-LPS remain to be identified.