Activities of imipenem and vancomycin against methicillin resistant Staphylococcus aureus (MRSA) were compared in vitro and in a rabbit model of aortic valve endocarditis. Against 25 MRSA clinical isolates, imipenem was bacteriostatic (MIC90/MBC90, mg/l 8/32) in vitro while vancomycin was bactericidal (MIC90/MBC90, mg/l 2/4). Rabbit endocarditis was produced with a MRSA isolate against which both drugs were bactericidal. Imipenem-cilastatin had better efficacy than vancomycin by the following criteria, the number of survivors (9/13 vs 7/13), clearance of bacteraemia (9/9 vs 3/7; P = 0.019), sterility of cardiac vegetations (9/9 vs 1/7; P = 0.001) and sterility of distant organs (8/9 vs 2/7; P = 0.035). Thus, imipenem-cilastatin may be a potentially useful alternative agent to vancomycin in the therapy of MRSA endocarditis in the occasional situations when the drug demonstrates in-vitro bactericidal activity against the pathogen. Efficacy against MRSA strains with higher MBCs remains to be proved.