To determine whether low concentrations of aminoglycosides in combination with penicillin could effectively kill enterococci in vitro, we tested penicillin (20 μg/ml) in combination with decreasing concentrations of either streptomycin (20, 10, 5, and 1 μg/ml) or gentamicin (5, 3, 1, and 0.5 μg/ml) against 13 strains of streptomycin-susceptible and 7 strains of streptomycin-resistant enterococci isolated from patients with infective endocarditis. At 24 h, penicillin plus each increment in streptomycin concentration resulted in a statistically significant increase in killing of streptomycin-susceptible enterococci, compared with the next lower streptomycin concentration (P<0.01). At 24 h, against streptomycin-susceptible and streptomycin-resistant enterococci, there were no statistically significant differences in killing between combinations containing 5 μg of gentamicin per ml and those containing 3 μg/ml. Against streptomycin-susceptible enterococci, there were statistically significant differences in killing between combinations containing 3 μg of gentamicin per ml and those containing 1 μg/ml. Against streptomycin-resistant enterococci, statistically significant differences in killing were detected with combinations containing 5 μg of gentamicin per ml and those containing 1 μg/ml.