The successful therapy of enterococcal endocarditis requires prolonged administration of synergistic antibiotic combinations. Controversy has arisen regarding optimal therapy when the organism possesses high level streptomycin resistance, and when the patient is allergic to penicillin. This study examines these questions in vitro and in a rabbit model of enterococcal endocarditis. The combination of penicillin with either streptomycin or gentamicin increased the rate of bacterial killing in vitro and in vivo when compared with penicillin alone (P < 0.05) when the test strain was relatively susceptible to streptomycin (minimal inhibitory concentration, 128 μg/ml). Only the combination of penicillin and gentamicin was consistently more effective than penicillin alone (P < 0.01) when the test strain was highly resistant to streptomycin (minimal inhibitory concentration > 150,000 μg/ml). The combination of vancomycin and streptomycin was more rapidly bactericidal than vancomycin alone in vitro and in the animal model against the streptomycin susceptible strain (P < 0.01). The relative rate of in vitro bacterial killing by various antibiotics and combinations was predictive of the efficacy of these drugs in eradicating enterococci from cardiac vegetation in experimental endocarditis.