The effects on plaque induced by the use of xylitol-and sucrose-containing chewing-gums were studied on ten test-persons with neglected oral hygiene during two three-day periods. The chewing of three, as well as of ten pieces per day of a xylitol chewing-gum resulted in a significant reduction in the amount of plaque and in the decrease of activity of invertase-like enzymes in plaque extracellular phase, when compared to corresponding conditions where a sucrose-containing chewing-gum was used.