Polydextrose is a randomly bonded condensation polymer of glucose, containing minor amounts of bound sorbitol and citric acid. This material has been developed for a reduced-calorie bulking, bodying or texturizing agent in the manufacture of low-calorie foods. The effect of polydextrose or refined preparation which has been eliminated saccharides under molecular weight of 500 in polydextrose, on cariogenicities of Streptococcus mutans group (serotype a-g) and certain oral microorganisms was studied. Some of S. mutans strains (serotype c and f) metabolized polydextrose at a low level and produced 20% lactic acid as compared with glucose. It did not serve as a substrate for plaque formation and cellular aggregation of S. mutans group and did not utilized water-insoluble glucan synthesis by glucosyltransferase from S. mutans PS-14 (c) or S. sobrinus 6715 (g). In the adherence experiment using polydextrose, heat killed cells of S. mutans labelled with 3H and GTase, the amount of cells that adhered to the smooth surface was almost 0. Further, polydextrose inhibited sucrose dependent water-insoluble glucan synthesis. The case of refined polydextrose was not utilized for the growth, acid production, cellular aggregation and plaque formation of Streptococcus mutans group (serotype a-g). Refined polydextrose also did not produce lactic acid by certain oral microorganisms. It did not serve as a substrate for water-insoluble glucan synthesis and cellular adherence by GTase from S. mutans PS-14 (c) or S. sobrinus 6715 (g). Further, refined polydextrose inhibited sucrose dependent cellular adherence by GTase from S. mutans PS-14(c). SPF SD rats infected with S. sobrinus 6715 were fed a diet containing 20% refined polydextrose or 20% sucrose for 48 days. A significantly (p less than 0.01) lower caries score (mean +/- SE, 4.74 +/- 0.86) was observed in the rats fed a diet containing refined polydextrose than the control (31.17 +/- 0.33). Also, the value of caries score of rats fed a diet containing the refined polydextrose was almost equal as that of the rats fed a diet containing cornstarch only.