Interspecies haematological differences to MS 222 and neutralized MS 222 anaesthesia were investigated in Sarotherodon mossambicus, Cyprinus carpio and Salmo gairdneri acclimated under identical laboratory conditions. Anaesthesia with MS 222 resulted in a ‘chemical stress’ in all fish, as was evident from changes in the haematological profiles of the animals. Such species specific variations in the haematology persisted throughout the whole experiment protocol which employed different concentrations of the anaesthetic. The use of neutralized MS 222, whereby aquarium water quality remained unchanged, improved the haematological profile. Possible reasons for the interspecies differences observed are discussed.