It was observed in a previous study that the growth of Streptococcus mutans strain OMZ 176 on sorbitol was inhibited by xylitol. The aim of the present study was to investigate the mechanisms involved in this inhibition. It was shown that the uptake of 14C-sorbitol was delayed when the cells had been pre-exposed to xylitol, and that the only labelled substance found intracellularly was sorbitol; no further metabolization occurred. This is in contrast with untreated normal cells, where sorbitol is taken up by a specific phosphotransferase system (pts). The 14C-xylitol metabolism of the cells was qualitatively unchanged in the presence of sorbitol; an intracellular accumulation of 14C-xylitol-phosphate (xylitol-P) and 14C-xylulose-phosphate (xylulose-P) was observed. However, a reduced uptake of xylitol was observed in the presence of sorbitol. Xylitol thus appears to change the pathway by which sorbitol is taken up by the cells. An inducible permease may replace the normal sorbitol pts when xylitol is present. No further metabolization of this intracellular sorbitol seemed to occur in the resting cell suspensions. It was furthermore observed that the presence of sorbitol enhanced the inhibitory potential of xylitol. The accumulation of intracellular sorbitol coincided with markedly increased xylulose-P/xylitol-P ratio. It may be speculated that, if xylulose-P were the major inhibitor of the glycolysis instead of xylitol-P, as previously assumed, an increased concentration of xylulose-P induced by sorbitol could explain that sorbitol enhances the inhibition potential of xylitol. It is not evident, however, how intracellular sorbitol could affect the xylulose-P/xylitol-P ratio.