The effects of added maltose on the activities of a preparation of crude glucosyltransferases (GTases) and purified dextransucrase (DS) were investigated to elucidate the inhibition mechanism of maltose on the synthesis of water-insoluble glucan (ISG) in Streptococcus mutans HS-6. Tri- and tetra-saccharides produced by crude GTases from sucrose in the presence of maltose were identified as panose (4-alpha-isomaltosylglucose) and 4-alpha-isomaltotriosylglucose which were responsible for the activity of DS involved in crude GTases. Kinetic studies on crude GTases in the presence of maltose showed similar results to those of DS except that the synthesis of ISG in the crude GTases was inhibited. Comparative studies of soluble products of crude GTases and DS in the presence of maltose were performed employing gel filtration on Sephadex G-15. The existence of oligosaccharides above hexasaccharide was revealed as the products of DS but not of crude GTases. These findings were interpreted in terms of the previously proposed mechanism of ISG synthesis by S. mutans, i.e., ISG should be synthesized from the preformed soluble glucan. It was indicated that oligosaccharides above hexasaccharide are utilized for ISG synthesis in the crude GTases system. From these results, the inhibitory mechanism of added maltose on ISG synthesis by crude GTases is considered as follows: DS synthesizes a series of 4-alpha-isomaltodextrinylglucose from sucrose and maltose, and the increase of added maltose results in the decrease of oligosaccharides responsible for synthesis of ISG.