The efficacy of imipenem alone or in combination with gentamicin for the treatment of experimental endocarditis in rabbits caused by a penicillin resistant strain of Streptococcus sanguis was compared to that of gentamicin and penicillin G. In vitro, both beta-lactams were bactericidal in action and the combination of gentamicin and imipenem was synergistic after 4 h whereas penicillin G and gentamicin showed synergy after 8 h. Antibiotic treatment was started two days after initiating infection and groups of 20 animals were treated for 5 days with imipenem alone or in combination with gentamicin or with gentamicin and penicillin G. Two of 20 rabbits which received no therapy died whereas all other rabbits survived the treatment period. Blood cultures became negative in 19 rabbits on the second day of treatment with imipenem and gentamicin and sterilization of the vegetations was achieved in 12 animals. In contrast, eight animals given the alternative treatments remained bacteraemic and viable organisms were recovered from the vegetations in every case. However, bacterial counts and vegetation weights only differed significantly between treated animals and controls but not between the different treatment groups. Never the less, these data suggest that combined treatment with imipenem and gentamicin might provide an effective regimen for patients with endocarditis caused by penicillin resistant S. sanguis.