Biomarkers include those biochemical, physiological, morphological, and histopathological responses of organisms that signify exposure to contaminants (Melancon 1995). More specifically, a biomarker is "a toxicant-induced change in gene expression leading to alteration in protein content and enzyme activity that is linked to the amount of contaminant" (Bucheli and Fent 1995b). Ideally, a biomarker can also "...estimate or predict chemical exposure and/or effects at higher levels of organization," thereby serving to link cause-and-effect between environmental exposure and biological responses (Kloepper-Sams and Benton 1994).

Bucheli, T. D. and Fent, K. Induction of cytochrome P450 as a biomarker for environmental contamination in aquatic ecosystems. Critical Reviews in Environmental Science and Technology. 1995; 25(3)201-268.
Kloepper-Sams, P. J. and Benton, E. Exposure of fish to biologically treated bleached-kraft effluent 2. Induction of hepatic cytochrome P4501A in mountain whitefish (Prosopium williamsoni) and other species. Environmental Toxicology and Chemistry. 1994; 13(9)1483-1496.
Melancon, M. J. Bioindicators used in aquatic and terrestrial monitoring. In Hoffman, D. J.; Rattner, B. A.; Burton, G. A.; Jr.; Cairns, J., and Jr. editors. Handbook of ecotoxicology. Boca Raton (FL) CRC Press; 1995; pp. 220-239.