This is the periodic examination of biological specimens for the purposes of monitoring their exposure to (or the effects of) potentially toxic chemicals in the environment.
This is normally done by analyzing the amounts of the toxic substances or their metabolites present in body tissues and fluids.
The term is also used to mean assessment of the biological status of populations and communities of organisms at risk, in order to protect them and to gain an early warning of possible hazards to human health.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.