Fresh plasma from Cryptobia-resistant brook charr (Salvelinus fontinalis) lysed Cryptobia salmositica under in vitro conditions. However, the parasite was not lysed if the plasma was heat-inactivated at 37 degrees C for 2 h or after the addition of ethylenediaminetetraacetic acid (EDTA) to chelate the free Mg2+ ions. Addition of Mg2+ ions to EDTA-chelated plasma restored parasite lysis. Treatment of Cryptobia-resistant plasma with either ethylenebis (oxyethylenenitrilo)tetraacetic acid, a Ca(2+)-chelating agent, or cobra venom factor did not reduce their lytic titres. The alternative pathway of complement activation is the mechanism of innate immunity against C. salmositica. The present study also shows that there is functional heterogeneity amongst the complement components activated via the alternative pathway within a species (brook charr) and between species (brook charr and goldfish) of fishes.