The response of the glutathione S-transferase (GST, EC 184.108.40.206) and related proteins of Mytilus edulis to environmental pollution load was assessed. Mussels were reciprocally transplanted between an industrial estuary (Douglas), a rural estuary (Youghal) and a marine site (Bantry). In addition, mussels were sampled along a pollution gradient in an estuary receiving leather tannery effluent (Colligan). These latter mussels were previously shown to be subject to oxidative stress resulting from the discharges. GST specific activity of cytosolic extracts from the gill and digestive gland tissues was determined for all animals. Specific activity was shown to vary significantly in animals from different sites, with highest specific activity always observed in samples (local and transplanted) taken from the industrial site. By comparison, the mussels exposed to tannery discharges displayed no significant alteration in GST specific activity. Total intracellular glutathione (GSH) was also determined for samples taken from the Douglas and Youghal estuaries but no correlation with pollution load was observed. Using FPLC analysis, we observed no specific effect on the relative levels of the GST and the individual GST related proteins in gill or digestive gland samples from local or reciprocally transplanted animals from Douglas or Youghal, The increase in GST specific activity observed in samples from the industrial estuary are indicative of a possible, specific inductive agent at this site. The results from the tannery site, by comparison, indicate that general oxidative stress does not result in elevated GST specific activity in M. edulis.