Groups of mice were infected with two strains of Salmonella typhimurium. Those infected with the less virulent strain no. 8 were treated with niridazole in doses of 200 or 600 mg per kg. Those infected with strain no. 12 received either 100 or 400 mg per kg. Compared with non-treated control mice the treated groups showed significantly higher numbers of survivors. In the group infected with strain no. 8 no difference was found in the incidence of survivors between the 200 and 600 mg per kg treatment groups. Such a difference was noted, however, in the case of strain no. 12 between the 100 and 400 mg per kg groups. Bacterial counts of spleens of survivors showed that treatment with niridazole significantly reduced the numbers of bacteria as compared with control mice.