The yellow mutagen, 3, 3'-di-tert-butyl-biphenyldiquinone-(2, 5, 2', 5'), which was formed from butylated hydroxyanisole when reacted with sodium nitrite under acidic conditions, was transformed into a red crystal by irradiation with sunlight in an ether solution. The photolytic product was identified as 2, 6-di-tert-butyl-8-hydroxy-dibenzofuran-l, 4-quinone from instrumental analyses. During the irradiation, two kinds of free radicals were detected by electron spin resonance spectroscopy. A photolytic reaction mechanism, in which radical intermediates participate, is proposed on the basis of the data obtained. The photolytic product showed potent desmutagenic activity to frameshift-type reverse mutations induced by Trp-P-1 and Trp-P-2 for a strain of Salmonella typhimurium TA 98.