Natural anti-proteases (α1-protease inhibitor (α1-PI; α1-antitrypsin) and α2-macroglobulin (α2-M)) were found in the blood of rainbow trout, Oncorhynchus mykiss and brook charr, Salvelinus fontinalis. The α2-M inhibited Cryptobia salmositica proteases and was significantly higher in brook charr than in rainbow trout. Under in vitro conditions it took longer for the same number of parasites to neutralize the α2-M in charr than in trout blood. The haemolysis which occurred when C. salmositica was incubated in the blood of rainbow trout was due to neutralization of α2-M. This in vitro study also showed that it was the metalloprotease of C. salmositica that lysed red blood cells and the plasma of the two species of fishes initially prevented haemolysis by inhibiting the proteolytic activity. We suggest that the natural plasma α2-M plays an important role in defence against cryptobiosis in fishes.