In this report, the author reports about the effect of combinative administrations of streptomycin, aureomycin and chloromycetin on the resistant strain of Salmonella typhi (S57S), and also about the relationship of cross-resistance among them: 1) To the 100γ/ml of streptomycin-resistant strain, the following combinative administrations are co-operative; streptomycin and aureomycin, streptomycin and terramycin, and streptomycin and chloromycetin. 2) To the 30γ/ml of aureomycin-resistant strain, the following combinative administrations are all co-operative; aureomycin and terramycin, aureomycin and streptomycin, and aureomycin and chloromycetin. 3) To the 10γ/ml of chloromycetin-resistant strain, the following combinative administrations are all co-operative; chloromycetin and streptomycin, chloromycetin and terramycin, and chloromycetin and aureomycin. 4) The streptomycin-resistant strain is not resistant to terramycin, aureomycin, and chloromycetin. 5) The aureomycin-resistant strain is not resistant to streptomycin, but is resistant to chloromycetin and terramycin. 6) The chloromycetin-resistant strain is not resistant to streptomycin, but is resistant to aureomycin and terramycin.