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Seventy-one clinical isolates of 5 species of group D Streptococcus (Enterococcus) were tested for susceptibility to 15 antibiotics at pH 5.0, 7.4, and 8.5. Penicillin G, ampicillin, cephalothin, cephaloridine, and novobiocin were considerably more active against all strains at pH 5.0 than in the more alkaline media. On the other hand, lincomycin, cindimycin, erythromycin, and gentamicin were moderately to markedly more active at pH 8.5. No important differences were noted in the susceptibility of the strains to kanamycin and streptomycin, at the pH levels tested, but the organisms were quite resistant to them in these tests. The various species differed quantitatively in their susceptibility to the individual drugs and in the effects ofpH. The size of the inoculum also had a variable effect on susceptibility, depending on the species of Enterococcus, the antibiotics, and the pH of the test medium. The data suggest that, in the antibiotic treatment of urinary tract infections caused by Enterococcus, attempts should be made to achieve the optimum pH in the urine, particularly in view of the fact that organisms of this group are often resistant to several antibiotics in the usual tests.