The article deals specifically with the application of biomarkers for two different purposes: to diagnose sublethal stress in an organism, designated as a tier I biomarker, and to detect exposure to specific contaminants, a tier II biomarker. Important criteria for evaluating the utility of tier I biomarkers for diagnosing sublethal stress would include: (1) its ability to be used in a broad range of organisms when exposed to a wide variety of stress conditions in their environment, (2) that it correlates with decreased physiological function and survival of the organism, and (3) that, in practicality, it can be easily measured in a cost efficient manner. Tier II biomarkers, used to identify exposure to specific contaminants, should be detected in organisms exposed to a particular class of contaminants in their environment and be easily measured. By integrating both kinds of biomarkers into a multitiered approach to environmental monitoring, one could develop a series of assays in which organisms are initially screened with biomarkers to detect general stress and, if the results were positive, could be assayed with an array of tier II biomarkers, each of which identifies exposure to a particular class of contaminants or physical conditions. (Copyright (c) 1990 by Lewis Publishers.)