The use of enzyme activity or concentration of biochemicals in the tissues to detect pollutant stress in fish is reviewed. Of the many biochemical parameters which have been measured, most are of limited value at present, primarily because correlations between biochemical effects and significant deleterious effects in the fish population have not been established. Characteristics of the metallothionein and mixed-function oxygenase systems and activity of acetylcholinesterase and delta amino levulinic acid dehydratase in fish tissues or blood may be useful for identifying fish which have been exposed to particular classes of pollutants. Some blood enzymes and biochemicals are useful for diagnosing liver damage in fish. Biochemical composition and concentrations in the blood and tissues of fish. Biochemical composition and concentrations in the blood and tissues of fish from polluted habitats differ in characteristic ways from those of fish of the same species from nearby clean areas. Some of these differences show promise for use in assessing damage to fish populations.