Homocitric acid tririboside which inhibited dextransucrase was isolated from a culture broth of Streptomyces sp. MF980-CFI.It has been reported that Streptococcus mutans plays an important role in the initiation of dental caries'), and the cariogenicity depends on its dextransucrase (EC 2.4.1.5) that produces from sucrose insoluble polysaccharides, which mediates bacterial adhesion to the tooth surface'-').We devised a screening method of inhibitors of dextransucrase (Fig. 1), and found a strong inhibitor in culture filtrates of the strain MF980- CFI. This is the first finding of a specific inhibitor of S. mutaus dextransucrase.We named this inhibitor ribocitrin, pertinent to its structure.As reported in the other paper'), ribocitrin has a unique structure consisting of three moles of ribose and one mole of homocitric acid.