Glucuronidation of three substrates prototypical for different UDP-glucuronosyltransferase (UDPGT) isoforms in hepatic, renal, intestinal, and branchial microsomes of corn oil, 3-methylcholanthrene, Aroclor 1254, and clofibrate-pretreated plaice was investigated. The differential expression of UDPGT in the four tissues clearly demonstrated for the first time that multiple isoforms with differing substrate specificities were present in fish. The liver was quantitatively the most important site for the glucuronidation of all three compounds studied. Phenol UDPGT activity was ubiquitous to all tissues and was induced by 3-methylcholanthrene and Aroclor 1254 in hepatic tissue and by Aroclor 1254 in renal tissue. The glucuronidation of testosterone was restricted to liver and intestinal tissue, while conjugation of bilirubin was expressed solely in hepatic tissue. The biotransformation of the endogenous compounds was not induced in the xenobiotic-treated animals. The presence of immunoreactive UDPGTs in the four tissues was demonstrated by immunoblot analysis using sheep anti-plaice UDPGT antibodies. Hepatic tissue displayed a range of immunoreactive polypeptides of 52 to 57 kDa, while a 55-kDa polypeptide was detected in extrahepatic tissues. An increased intensity of the latter polypeptide species was demonstrated in liver and kidney microsomes in which there was a concomitant induction of phenol UDPGT activity in xenobiotic-treated fish. The results indicate that the 55-kDa polypeptide was the major polyaromatic hydrocarbon-inducible UDPGT isoenzyme in hepatic and renal microsomes.