Evidence is presented in support of the thesis that the ubiquitous metal-binding protein, metallothionein (MT), may serve as a biomonitor of exposure to environmental toxins such as heavy metals and to environmental stress. MT has been demonstrated to be induced by exposure of vertebrates, invertebrates, and microorganisms to elements of toxic potential such as cadmium, platinum, gold and mercury, and by the stresses of restraint and food and water deprivation. Methods have been developed to permit accurate and sensitive detection and quantitation of MT in mammals, including radioimmunoassay (RIA) and enzyme- linked immunosorbent assay (ELISA), both assays based on the use of both polyclonal and monoclonal antibodies to the protein. Panels of specific antibodies may be readily produced to monitor MT levels in a selected series of nonmammalian sentinel species.