In this report, the author reports about the effect of the combinative administrations of each two of the following antibiotics, penicillin, streptomycin, aureomycin, terramycin, and chloromycetin; the tested combinations were 10 as a whole. As a test organism, Staphylococcus aureus (Terashima) was taken. The results are as follows: 1) The antibacterial actions are co-operative to each other in each of the following combinations, penicillin and streptomycin, aureomycin and terramycin, aureomycin and chloromycetin, and chloromycetin and terramycin. 2) Though very slight, the antibacterial actions are co-operative in each of the following combinations, streptomycin and terramycin, streptomycin and aureomycin, and streptomycin and chloromycetin. 3) In each of the combinations of penicillin with chloromycetin, terramycin, and aureomycin, the antibacterial action of penicillin is inhibited by the other, while each action of chloromycetin, terramycin, and aureomycin is enhanced by penicillin. 4) Among the groups of tested organism, the individual difference of resistance to antibiotics is observed.