Biotransformation of sinesetin by Aspergillus niger afforded 4'-hydroxy-5,6,7,3'-tetramethoxyflavone on the basis of its spectroscopic data including IR, heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond connectivity (HMBC) analysis. Sinesetin and the metabolite showed antimutagenic activity against chemical mutagens 4-dimethyl-3H-imidazo[4,5-f]quinolin-2-amine (MeIQ) and 3-amino-14-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) in the umu test using Salmonella typhimurium TA1535/pSK1002. (c) 2005 Society of Chemical Industry.