Biological monitoring of exposure to an industrial chemical may be defined as the evaluation of the internal dose (internal exposure) by a biological method with the view of assessing the associated health risk. Depending on the biological parameter selected and on the time of sampling, internal dose may mean the amount of chemical recently absorbed, the amount stored in one or several compartments of the organism, or, under ideal conditions, the amount bound to the sites of action. The estimation of the internal dose has usually relied on two categories of tests: those based on the determination of the chemical and/or its metabolites in various biological media (eg, alveolar air, blood, urine) and those based on the quantification of a nonadverse biological effect, the intensity of which is related to the internal dose. Biological tests providing information on the amount of active chemical bound to the critical sites of action (target dose) and noninvasive analytical methods which measure the amount of cumulative toxins stored in certain body compartments (eg, bone, liver, kidney) are currently being developed. The main criteria to be considered in the selection of the most appropriate biological monitoring test of exposure are the specificity of the parameter and its sensitivity with respect to the exposure level, the analytical and biological applicability, the convenience for the workers, the possible risks associated with its application, and, of course, the accuracy of the exposure and/or health risk evaluation. Normally, the biological approach of exposure monitoring is reserved for chemicals which enter the organism and may exert systemic toxic effects. When a biological monitoring test of exposure is applicable, it offers several advantages over ambient air monitoring. A biological parameter reflecting the internal dose is necessarily more closely related to the systemic adverse effect(s) than any environmental measurement. Biological monitoring of exposure integrates the exposure by all routes (pulmonary, oral, and cutaneous) and sources. It may also take into account several factors which determine the uptake and the absorption of the chemical (individual variability, work load, hygiene habits, etc). However, contrary to ambient monitoring, biological monitoring is applicable only when sufficient information has been gathered on the metabolism of the chemical, its toxicity or that of its metabolites, and when some of the relationships between external exposure, internal dose and adverse effects have been assessed.(ABSTRACT TRUNCATED AT 400 WORDS)