Thirty strains of Enterobacteriaceae and ten strains ofPseudomonas aeruginosawere selected to study thein-vitroactivity of various combinations containing aminoglycosides, by the checkerboard method. The following aminoglycosides: gentamicin, dibekacin, netilmicin and amikacin were used in combination with each of the following: ampicillin, ticarcillin, piperacillin, cefazolin, cefuroxime, ceftazidime, norfloxacin, enoxacin and minocycline. Synergy was not a constant feature ofβ-lactam+aminoglycoside combinations. Discrepancies occurred with the same strain submitted to combinations containing different aminoglycosides. The combinations containing new quinolones+aminoglycosides were usually additive, although occasionally synergy occurred. The combination minocycline+aminoglycoside was usually additive but partial antagonism (FIC or FBC index: 1·5–<2) occurred in 10 to 40%of the strains depending on the aminoglycoside. The combinations ofβ-lactams with dibekacin or netilmicin most frequently produced partial or full synergy against strains ofPs. aeruginosasusceptible to all four aminoglycosides te