A forward mutation assay has been developed in Salmonella typhimurium. The L-arabinose-sensitive strain SV3 of Salmonella typhimurium is being calibrated against mutagens causing base-pair changes, frameshift mutations, and deletions. New strains lacking the excision-repair system and/or the lipopolysaccharide barrier were selected from SV3. The sensitivities of the new strains have been compared with strain SV3. Physiological and biochemical studies will be performed with the strains in order to characterize the L-arabinose-resistance mutagen assay.