The inhibitory and bactericidal effects of gentamicin, amikacin, netilmicin (Sch 20569), and carbenicillin were tested against 55 clinical isolates of Serratia marcescens that had been subtyped into 26 strains by biotyping and serotyping. Three major patterns of resistance of gentamicin, netilmicin, and carbenicillin were recognized among these isolates. (i) Most of the 27 isolates that were susceptible to gentamicin (minimal bactericidal concentration [MBC] ≤ 6.25 μg/ml) were susceptible to carbenicillin (MBC ≤ 125 μg/ml) and resistant to netilmicin (MBC ≥ 12.5 μg/ml). (ii) Most of the 11 isolates with moderate resistance to gentamicin (MBC of 12.5 to 25 μg/ml) were also susceptible to carbenicillin and resistant to netilmicin. (iii) The 17 isolates with high level resistance to gentamicin (MBC ≥ 50 μg/ml) were all highly resistant to carbenicillin (MBC ≥ 8,000 μg/ml) but susceptible to netilmicin (MBC ≤ 6.25 μg/ml). The susceptibility to amikacin was unpredictable among these groups of isolates but, overall, 80% of the isolates were killed by 25 μg of amikacin/ml, which is within the range of peak serum concentrations used therapeutically. Clinically attainable subinhibitory concentrations of carbenicillin enhanced the activity of the three aminoglycosides against all isolates with MBCs of carbenicillin ≤ 2,000 μg/ml. The 17 isolates with high level resistance to carbenicillin and gentamicin, as well as the four isolates with high level resistance to carbenicillin but not to gentamicin, were not susceptible to such enhancement of aminoglycoside activity by carbenicillin.