A clinical trial to test the compound glycyrrhizin, the sweet component of licorice, was conducted on 21 dental students. A split-mouth technique of glycyrrhizin application was used. Subjects were instructed to discontinue all oral hygiene procedures, but no dietary modifications were imposed. After 3 days a highly significant reduction in plaque was detected in the upper central incisors on the experimental sides compared with the control sides of students' mouths. Comparing all teeth, less plaque was found on experimental sides than on control sides of the mouths. This difference demonstrated a tendency towards statistical significance. After 4 days the quantitative differences between the two halves of the mouths (less plaque on experimental sides) were greater than after 3 days. This pilot study might indicate the potential of glycyrrhizin in controlling dental plaque.