We evaluated the in vitro synergistic activity at clinically attainable concentrations of combinations of aminoglycoside and beta-lactam antibiotics against 30 gentamicin-resistant clinical isolates of gram-negative bacilli. All 56 pairs of 4 aminoglycosides and 14 beta-lactams were evaluated. Combinations with amikacin demonstrated inhibitory synergistic activity in 29% of the assays, as compared with 22% for netilmicin (P = 0.018), 17% for gentamicin (P less than 0.001), and 13% for tobramycin (P less than 0.001). Among the beta-lactams, combinations with cefoperazone, ceftriaxone, or cefpiramide (SM-1652) demonstrated inhibitory synergistic activity most often (39, 38, and 35% of the assays, respectively) and with ceforanide, cefsulodin, and imipenem least often (less than or equal to 8% each). The most active combination was amikacin and ceftriaxone, with which 67% of the assays demonstrated inhibitory synergism. Isolates with high-level resistance to either antibiotic in a combination were unlikely to be inhibited synergistically by the combination. Further, combinations generally demonstrated little synergistic activity against isolates highly susceptible to beta-lactams.