Combinations of EDTA-tromethamine and 7 antimicrobial agents [chloramphenicol, nalidixic acid, oxytetracycline, penicillin, polymyxin B, streptomycin, and triple sulfa (sulfamethazine, sulfapyridine, and sulfathiazole]) were tested for synergistic activities against Escherichia coli. Three in vitro tests were used including minimal inhibitory concentrations of the drugs, a 2-dimensional microtiter checkerboard technique, and bacterial inhibition studies. A strong synergistic inhibitory action was observed with combinations of EDTA-tromethamine plus penicillin and EDTA-tromethamine plus oxytetracycline. Combinations of EDTA-tromethamine plus chloramphenicol also had a synergistic action, but to a lesser extent than that with penicillin and oxytetracycline. When streptomycin or nalidixic acid was mixed with EDTA-tromethamine, a slight synergistic action was noticed. There was no synergistic effect recorded with combinations of EDTA-tromethamine and triple sulfa. Combinations of EDTA-tromethamine and polymyxin B had an antagonistic effect.