Studies with isolated plaice hepatocytes incubated with benzo(a)pyrene showed that glucuronidation was the major phase II pathway and glutathione conjugation contributed significantly, whilst sulphation was very low. Molecular studies of plaice (Pleuronectes platessa) and flounder (Platichthys flesus) UDPGT and GST enzymes were undertaken. Immunoblotting studies have shown that both flatfish livers contain multiple isoforms of both UDPGT and GST which have been confirmed by enzyme purification and cDNA cloning studies. The effects of intraperitoneal injection of the prototypical inducers 3-MC, ARO and TSOX are described, with particular reference to mRNA, polypeptide and catalytic activity levels of GST and UDPGT isoenzymes. Plaice hepatic phenol UDPGT activity was induced by planar aromatic hydrocarbons. In contrast both 3-MC and ARO repressed the mRNA and polypeptide homologue of GST-A in flounder liver but induced CDNB conjugation indicating that GST isoforms may be differentially regulated. The epoxide TSOX strongly induced GST-A, its mRNA and also CDNB conjugation activities in flounder liver suggesting a functional role in detoxification of reactive cytotoxic radicals rather than in excretion of PAH metabolites.