Mutastein, a new inhibitor of glucosyltransferases (GTase) of Streptococcus mutans, was isolated from cultural broth of Aspergillus terreus M3328. Mutastein was a heat-stable protein and was sensitive to pronase digestion. In the formation of glucans, that of adhesive water-insoluble glucans of S. mutans was specifically inhibited by mutastein.