The mechanism by which chlorhexidine kills bacteria is still ill defined. We have investigated the action of chlorhexidine on Escherichia coli JM101/psb311 using a combination of flow cytometry and traditional methods. Chlorhexidine-induced uptake by E. coli cells of bis-(1,3-dibutylbarturic acid) trimethine oxonol and propidium iodide, which monitor membrane potential and membrane integrity respectively, was shown to be concentration dependent for the range 0.003-0.3 mmol-1. In addition, cells in log phase growth were more susceptible to 0.03 mmol-1 chlorhexidine than those in stationary phase. There was, however, no direct correlation between dye uptake and decline in colony forming units.