Microsomal UDPglucuronosyltransferase(1-naphthol), an enzyme form previously shown to be selectively inducible in rat liver by 3-methylcholanthrene-type inducers, was purified to apparent homogeneity. Rabbit antibodies against this enzyme form precipitated UDPglucuronosyltransferase activities towards 1-naphthol and 4-methylumbelliferone faster and to greater extents than enzyme activities towards bilirubin, oestrone and 4-hydroxybiphenyl. Ouchterlony double-diffusion analysis showed immunochemical similarity of the rat liver enzyme with the enzymes from other organs of the rat (kidney, testes) and the mouse liver but not with the enzyme from cat and human liver. Electroimmunochemical quantification of the enzyme indicated that its level was enhanced 1.3-fold and 2.5-fold in liver microsomes from phenobarbital-treated and 3-methylcholanthrene-treated rats, respectively. The results indicate that 3-methylcholanthrene treatment increases the enzyme level of rat liver microsomal UDPglucuronosyltransferase(1-naphthol). Despite phospholipid-dependence of its catalytic activity microsomal enzyme activity appears to be a good index of the enzyme level.