The possibility that chlorhexidine is a specific inhibitor of membrane bound bacterial adenosine triphosphatase (ATPase) was addressed. The in-vitro susceptibilities of several Providencia stuartii cell envelope enzymes, including ATPase, to chlorhexidine were compared. The following concentrations of chlorhexidine were required to cause 50% inhibition of enzyme activity in preparations from chlorhexidine-sensitive strains (MIC 50 mg chlorhexidine/l): ATPase (160 mg/l), succinic dehydrogenase (greater than 300 mg/l), penicillin binding protein 7 (300 mg/l) and beta-lactamase (45 mg/l). Fifty per cent inhibition of the ATPase from a chlorhexidine-resistant strain (MIC 1600 mg/l) was achieved at an in-vitro concentration of 225 mg chlorhexidine/l. Our observations do not support the suggestion that bacterial membrane-bound ATPases are specific targets for chlorhexidine.