Acinetobacter caleoaceticus var. anitratus is a nonfermentative, gram-negative bacillus that has been demonstrated to cause severe infections, usually in hospitalized patients. Since mild to moderate resistance of A. calcoaceticus to one or more aminoglycosidic aminocyclitols has been noted to occur, a study was undertaken to evaluate the activity of combinations of carbenicillin with either kanamycin, tobramycin, or gentamicin against 28 isolates of A. calcoaceticus obtained from clinical sources. Synergism (defined as at least 100-fold-increased killing at 24 h by the combination as compared with the most efficacious of the individual antibiotics) was demonstrated against 26 of 28 strains of A. calcoaceticus with carbenicillin plus kanamycin and carbenicillin plus tobramycin and against 25 of 28 strains with carbenicillin plus gentamicin. The median increased killing for the 28 strains was 4.2 log 10 with carbenicillin plus kanamycin and with carbenicillin plus tobramycin and 3.1 log 10 with carbenicillin plus gentamicin. The most important determinant of synergistic potential of each combination was the level of resistance of each strain of A. calcoaceticus to the aminoglycoside component of the combination.