The hepatic microsomes derived from various animal species transformed emodin (1,3,8-trihydroxy-6-methylanthraquinone), an anthraquinoid pigment present in fungal metabolites and a constituent of plant medicines, into an unidentified anthraquinone h, along with 2-hydroxy-, 4-hydroxy- and 7-hydroxyemodins. TLC, UV, MS and NMR clarified this unidentified major metabolite as omega-hydroxy-emodin (1,3,8-trihydroxy-6-hydroxymethylanthraquinone). Among 7 animal species, the highest activity to produce this omega-hydroxyemodin was observed in the hepatic microsomes of guinea pig and rat, followed by mouse and rabbit. The microsomal activity to convert emodin into omega-hydroxyemodin was accelerated by the pretreatment of animals with phenobarbital, and inhibited by SKF 525A. The microsomal hydroxylation reactions of the methyl residue and the anthraquinoid nucleus of emodin were presumed to be catalyzed regiospecifically by multiple forms of cytochrome P-450. omega-Hydroxyemodin was not mutagenic to Salmonella typhimurium in the absence of S9, but exhibited mutagenicity in the presence of an activating system. This genotoxic potential was comparable to 2-hydroxyemodin, a direct-acting mutagen.