Functional and molecular probes are described which are useful to identify a 3-methylcholanthrene-inducible phenol UDP-glucuronosyltransferase (GTMC) from rat liver. Two different procedures for isolation of GTMC were compared, method 1 utilizing DEAE-Sepharose chromatography or method 2, chromatofocusing. Method 2 appeared to be superior in separating different isoenzymes. Subsequently the enzyme was purified by affinity chromatography on UDP-hexanolamine Sepharose. With both methods a protein was purified with a subunit Mr of 55,000, catalyzing glucuronidation of a variety of planar phenols and, in particular, of benzo(a)pyrene-3,6-quinol to its mono- and diglucuronide. Antibodies to GTMC recognized a polypeptide with a subunit Mr of 55,000 as the major 3-methylcholanthrene-inducible isoenzyme in rat liver microsomes. The described functional and molecular probes may help to differentiate GTMC from similar isoenzymes conjugating planar phenols and to elucidate its regulation and biological function.