1.1. A series of in vitro bacteriostatic and bactericidal sensitivity tests have been carried out on 10 strains of Bact. coli using chloramphenicol, oxytetracycline, and streptomycin, and paired combinations of all three antibiotics.2.2. These tests have shown that, although the single and combined antibiotics exerted marked bacteriostatic effects, the combinations containing streptomycin produced better bactericidal actions.3.3. Passage experiments, using one antibiotic at a time in vitro, showed that varying degrees of resistance, markedly to streptomycin, less to chloramphenicol, and still less to oxytetracycline, could be developed. When paired combinations of antibiotics were used in the tests, least resistance developed when the various strains were passaged in streptomycin and oxytetracycline.4.4. A series of in vitro bacteriostatic and bactericidal sensitivity tests on 15 strains of B. proteus have shown that though chlortetracycline, chloramphenicol, and oxytetracycline had quite considerable bacteriostatic powers, they had little or no bactericidal action even in concentrations of 100 μg per milliliter. Streptomycin was more effective bactericidally but the combination of this antibiotic with one of the other antibiotics produced only a slightly more effective action.5.5. A series of 111 cases of urinary tract infections associated with obstetrical and gynecological conditions have been treated with oxytetracycline, combined oxytetracycline and streptomycin, and combined chloramphenicol and streptomycin.6.6. In 95 cases of Bact. coli infection, all three forms of medication were effective, but the combination of streptomycin and oxytetracycline seemed to produce more rapid improvement and in certain cases in which oxytetracycline or combined streptomycin and chloramphenicol had failed combined streptomycin and oxytetracycline effected a cure.7.7. In a small series of infections with B. proteus and Ps. pyocyanea, treatment with combined streptomycin and oxytetracycline gave more satisfactory results than any other form of antibiotic therapy.