Intermediary products in the synthesis of lavendamycin were tested for mutagenic activities in Salmonella typhimurium TA 98 and TA 100 with and without a metabolic activation system. Lavendamycin analogs having a methyl group at the 3' position showed a significant mutagenicity to TA 100 after the metabolic activation using S9 mix prepared from rat liver homogenate. Oxidative products of the 8-hydroxyguinoline derivatives were mutagenic without the metabolic activation. Of these oxidative products, desamino-desmethyllavendamycin methyl ester was identified as a metabolic product obtained by the incubation of the 8-hydroxyguinoline derivative with mouse liver homogenate.