Cloned cDNAs coding for hepatic UDP-glucuronyltransferase (UDPGT) have been isolated from a rat liver cDNA library in the expression vector bacteriophage lambda gt11 using anti-UDPGT antibodies. Four different mRNAs have been identified by sequencing of 15 UDPGT cDNA clones. The sequences of the four classes of cDNA were determined to be 85-95% homologous. Restriction fragments were isolated from the cDNA in each class and used as class specific probes. Hybridisation of these probes to northern blots of total RNA prepared from the livers of normal and genetically deficient Wistar rats identified the cDNA in class 4 with androsterone UDPGT. Translation of the cDNA sequence of clone rlug 23, the longest member of class 4, allowed determination of the complete amino acid sequence of androsterone UDPGT.