Haemolysis is associated with the destruction of Cs, and the formation of methaemoglobin indi- cates a change to the ferric (Fes state. Both haemo lysis and methaemoglobin formation diminish the oxygen-carrying capacity of blood. Little is known about the toxic effect of parasiticides in relation to cell damage associated with RBCs in fish. Hydrogen per- oxidase (H202), which is used to control the monoge- netic trematode, Benedenia seriolae, on yellowtail (Sako 1995), induced in vitro haemolysis of 75% and met haemoglobin of 60% at low concentrations of 83 ppm (Kawatsu et al. 1991) in common carp (Cypri- nus carpio). In the present study, formalin did not induce in vitro haemolysis at any of the test concentrations, but methaemoglobin formation was observed at concentrations above 250 ppm. This sug- gests that the mechanism of cell damage caused by formalin might be different compared with that caused by H202