This study investigated whether biotransformation and antioxidant enzymes of an organism tolerant to pollution such as the zebra mussel Dreissena polymorpha can be employed to evaluate the extent of urban water pollution. Activity changes of soluble and membrane bound glutathione S-transferase (s- and mGST), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) to environmental stress were explored in precultured mussels exposed for one day to one week at selected sites with reasonable anthropogenic impact. The enzymatic response of whole mussel tissue was compared to gill tissue. Changes in enzyme activity were accordant with the different pollution scenarios, as charges with polychlorinated biphenyls (PCB), dichlorodiphenyltrichloroethane (DDT) and other pesticides, as well as contamination with metals. Whereas in whole mussel tissue all analyzed enzymes responded with elevation, in gills inhibition took place with the exception of mGST. The results confirm the high sensitivity of gills, nevertheless enzymatic changes measured in whole mussel tissue provided the clearest results. Significant changes in GST, CAT and GPx enzyme activity were only observed at water temperatures above 20 degrees C. SOD activity was not restricted by temperatures and serves for this reason as a biomarker for oxidative stress at lower water temperatures. Discrepancies between biological and chemical evaluation of the sampling sites are presented and biomarker responses appraised.