The PH-value of B.T.B. PH-paper is not exact but that of P.R. (Phenol Red) PH-paper is exact in inorganic salt solution or synthetic liquid media, which contains more than 2000 μg/ml Streptomycin. Because, B.T.B. binds to Streptomycin in existence of the trace inorganic salt, and forms insoluble salt and cannot become to express the PH-changes. P.R. does not bind to Streptomycin.