Infectious disease caused by methicillin-resistant Staphylococcus aureus (MRSA) often develops in compromised hosts in whom a single administration of vancomycin is usually not effective. We assessed the combined antimicrobial effects of vancomycin and imipenem against MRSA growth in vitro as well as in vivo using a neutropenic mouse thigh infection model. Synergic and additive effects of the two drugs were observed for 34 and two, respectively, of the 36 clinical isolates of MRSA, as determined by the chequerboard method. For MRSA strain N, postantibiotic effect (PAE) values obtained in vitro were 1.9-2.6 h for vancomycin and 2.6-3.5 h for imipenem, while higher values of 2.7-4.4 h were obtained for the combination of vancomycin and imipenem. In vivo, a single administration of vancomycin at 1 or 2 mg/kg or of imipenem/cilastatin at 5 mg/kg produced growth curves similar to those in controls, with a suppressive time of 0 h. Imipenem/cilastatin at 10 mg/kg demonstrated a suppressive time of 3.1 h. Combinations of vancomycin 1 mg/kg plus imipenem/cilastatin 5 mg/kg, vancomycin 1 mg/kg plus imipenem/cilastatin 10 mg/k and vancomycin 2 mg/kg plus imipenem/cilastatin 10 mg/kg demonstrated suppressive times of 2.9, 3.9 and 5.2 h, respectively, indicating that combined administration was effective. Simultaneous administration of the two drugs was more effective than sequential administration. Thus, combined administration of vancomycin and imipenem/cilastatin proved effective for MRSA in vivo. The combined effects seemed to be synergic, since a single administration of either drug did not show anti-MRSA effects at the doses used in the combined regimen.