In the present study the bioaccumulation of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) as well as the responses of a suite of biochemical parameters were investigated in a standardized carp (Cyprinus carpio) fish line. Carps were caged for 2 to 8 weeks at two Amsterdam freshwater sites: the relatively unpolluted Outer IJ and the heavily polluted Volgermeerpolder. The primary objective of the experiments was to develop a reliable and reproducible method to monitor the inland water pollution and to classify the environmental quality of freshwater sites. The highest tissue levels of organic trace pollutants were generally found in carp that was caged for at least four weeks at the polluted site. Hepatic phase I enzymes showed the highest pollution-induced responses, while phase II enzymes and serum transaminases were less sensitive. The antioxidant enzymes showed virtually no response to pollutants. In order to assess effects on the balance between bioactivation and detoxication the ratio between phase I (EROD) and phase II (GST or UDPGT) activities was introduced as a biotransformation index (BTI). Highest BTI values were observed in carp that was caged for 4 to 6 weeks at the polluted site. The BTI values decreased after eight weeks of exposure due to elevated phase II activities.