This study compared the duration of serum bactericidal activity for vancomycin, 1 g every 12 or 24 h at steady state, against methicillin-resistant Staphylococcus aureus (MRSA) and coagulase-negative staphylococci (MR-CNS). All four test isolates were susceptible to vancomycin with minimal inhibitory concentration (MIC) values of either 2 or 4 mg/l. Serum bactericidal titres (SBTs) were run in duplicate and serum bactericidal activity (SBA) was defined as the time points at which all subject SBTs were greater than or equal to 1:2. For the every 12-h regimen, SBA was 10-12 h. With the every 24-h regimen, the duration of SBA was 10-16 h for MRSA and 8-10 h for MR-CNS. The pharmacodynamic data suggest that for those with good renal function a Q12h dosing interval is most appropriate for MR-CNS or staphylococcal isolates with MICs of 4.