The effect of chlorhexidine on the development of plaque, resulting from the inoculation with saliva of a tooth mounted in an artificial mouth, has been studied. The agent delayed the formation of plaque and also inhibited changes in pH and Eh, whether sucrose was present or not. Applied as a rinse at intervals of 12 h it prevented formation of plaque and pH and Eh remained constant. Moreover, chlorhexidine inhibited pH and Eh changes in established plaque. A single application of a gel containing chlorhexidine also greatly inhibited plaque development.