Mixed function oxidase (MFO) dependent biotransformation of polychlorinated biphenyls (PCBs) was measured in three different fish species from the North Sea. Liver microsomes of plaice (Pleuronectes platessa), dab (Limanda limanda) and cod (Gadus morhua) were isolated and incubated with different PCB congeners. Metabolites of PCBs were identified as monohydroxylated PCBs. Hydroxylation took place in meta- or para-position of the PCB molecules, preferential in para-position. Metabolization and metabolic rates depended on the substitution pattern of PCBs and the degree of chlorination. Biotransformations were also species dependent. The flatfish dab and plaice exhibited higher metabolic rates than cod (C) 1999 Elsevier Science Ltd. All rights reserved.