Proteases were detected in Cryptobia salmositica, C. bullocki, and C. catostomi. Maximum activities occurred in acidic pH and the overall activity in the pathogenic strain of C. salmositica was significantly higher than those in its nonpathogenic strain and the nonpathogenic C. catostomi. Cysteine protease was present in all three Cryptobia spp., but metallo-protease was only in the pathogenic C. salmositica. The pathogenic C. salmositica lost its metallo-protease after 10 months of in vitro culture. Metallo- and cysteine proteases were isolated from the crude extract of the pathogenic C. salmositica and the 200 kDa metallo-protease was purified. Both the metallo- and cysteine proteases had high activities against haemoglobin, fibrinogen and azocasein. The metallo-protease had high activities against gelatin and azocoll, while the cysteine protease had high activity against albumin. The optimal pH's for the metallo-protease and the cysteine protease were 7.0 and 5.0, respectively. The metallo-protease was secreted by the pathogenic C. salmositica and the secretion was significantly enhanced by collagen. The purified metallo-protease lysed erythrocytes from rainbow trout and digested red cell membrane proteins. It also completely degraded collagen and laminin. The pathogenic C. salmositica infected both rainbow trout (Oncorhynchus mykiss) and brook charr (Salvelinus fontinalis), but it caused anaemia in trout and not in charr. Natural antiproteases, $\alpha1$-protease inhibitor ($\alpha1$-PI) and $\alpha2$-macroglobulin ($\alpha2$-M), were identified in blood of trout and charr. The $\alpha2$-M inhibited the C. salmositica metallo-protease and the inhibition was significantly higher in charr than in trout. The in vitro neutralization of $\alpha2$-M was related to the haemolysis detected when live C. salmositica was incubated in the blood of trout. The onset and persistence of anaemia in infected trout was also related to its low $\alpha$2-M level. Infected brook charr did not develop anaemia and its $\alpha$2-M level remained high during the infection. The metallo-protease released by the pathogen was detected in the blood of infected trout at the time when its $\alpha$2-M level was low.