Streptomyces sp. KM1-30 was isolated from soil as a producer of antimutagens by screening with a modified Ames test. The chemical structure of the antimutagenic metabolite was identified as streptovaricin C, which is known to inhibit DNA dependent RNA polymerase from E. coli and RNA dependent DNA polymerase from RNA tumor viruses, by MS and 1H-, 13C-NMR analyses. Addition of streptovaricin C to the cultures of UV treated Salmonella typhimurium TA100 or Trp-P-2-treated S. typhimurium TA98 decreased the frequency of mutation without a decrease in viable cell counts. The effect of streptovaricin C to the mutation induced by UV and Trp-P-2 was not desmutagenic, but antimutagenic.