In an electrophoretic survey of lactate dehydrogenase (LDH) isozymes in neotropical cichlid fishes (Perciformes: Cichlidae) several species were discovered in which a cathodal liver-restricted isozyme is expressed along with the highly anodal eye-restricted isozyme (LDH-C,) typically encountered in perciform fishes. Biochemical characterization of these two isozymes from the basketmouth cichlid, Acuroniu nnssu (Heckel), strongly suggested that they were non-orthologous and challenged the accepted view that eye-and liver-restricted LDH isozymes are alternative expressions of the same (LDH-C*) gene. In this study, antiserum raised against cypriniform (goldfish) liver-restricted LDH-C, failed to cross-react with the basketmouth liver-restricted analogue while effectively titrating the eye-restricted, anodal isozyme and, at higher titres, the LDH-9, heart-restricted isozyme from all cichlid species. Anti-serum raised against basketmouth muscle-restricted LDH-A, failed to titrate any of the eye-and liver-restricted isozymes. These data confirm the orthology of eye-and liver-restricted LDH isozymes in Cypriniform and Perciform fishes as originally proposed, suggest that the liver-restricted isozyme of cichlid fishes is non-orthologous and further raise the question of identity and evolutionary origin of this anomalous LDH activity.