Silicone rubber catheters were experimentally colonised with three different strains of Staphylococcus epidermidis sensu stricto. The catheters were then perfused with brain-heart infusion broth by means of an in vitro apparatus which simulates the pharmacokinetics of intraventricular administration of antimicrobial agents. When colonisation was established, vancomycin was administered to the dosing chamber three times daily so as to give peak concentrations of 16, 32, 64 and 128 mg/l. Each experiment was performed in triplicate. Samples of fluid taken from each catheter were cultured before treatment and at intervals thereafter during perfusion for 4 weeks or until recolonisation became apparent. Those catheters not showing evidence of recolonisation were examined by scanning electron microscopy for the presence of organisms. Colonisation was eradicated (as judged by cultural examination) in 37% catheters but eradication did not bear any clear relationship to drug concentrations. Electron microscopical examination of all catheters showing eradication by cultural means revealed the presence of organisms.