Salmonella typhimurium TA1530 and G46 strains growing in minimal medium supplemented with sodium azide produce a stable mutagenic metabolite which is not azide. The production of this metabolite is restricted to the log phase of bacteria grown in the presence of azide. The metabolite is highly mutagenic in DNA-repair defective base-substitution strains TA1530 and TA1535, but ineffective in frameshift strains TA1538 and TA1537. The metabolite induces mutations in resting cells of the TA1530 strain.