The GSTs of M. edulis provide an easily assayed activity which would be expected to respond to changes in pollution status. The main GST and a related GSH-binding protein have been purified and biochemically characterized. The former protein is most similar to the Pi class while the latter is a catalytically inactive monomer which appears to be related to the Mu class. This enzyme activity has been assessed as a potential indicator of exposure to chemical pollutants in both Cork Harbour and Venice Lagoon (using the closely related species, M. galloprovincialis). Controlled exposure studies with mussels in holding tanks have indicated that the herbicide aldicarb gives a slight but significant increase in GST activity consistent with the inducibility of these enzymes by xenobiotics in this bivalve. At present, we are studying samples which have been deliberately exposed to PAH and PCB compounds. Studies of this type are important in helping to understand the effects and fate of chemical pollutants released into estuarine environments.