Saccharin markedly decreased the sorbitol- and mannitol-dependent growth rate and growth yield of Streptococcus mutans NCTC 10449 and also decreased the amount of acid produced from these hexitols by the organism. In addition, saccharin increased both the lag phase when glucose-grown cells were placed in a medium which contained sorbitol or mannitol as the primary energy source and to also increase the diauxie portion of the growth-response curve when glucose-grown cells were placed in a medium which contained a mixture of glucose and sorbitol or mannitol as fermentable energy sources. One mechanism by which saccharin may exert its negative effect upon hexitol catabolism by S. mutans is through inhibiting the activity of two inducible NAD-dependent oxidoreductases, sorbitol-6-phosphate dehydrogenase and mannitol-1-phosphate dehydrogenase which are found at high specific activities in extracts obtained from sorbitol- or mannitol-adapted cells respectively. Saccharin exerted a negative effect upon the enzymes by competitively inhibiting their interaction with both their oxidized and reduced coenzymes, NAD and NADH. In contrast, the inhibitor had no effect on the interaction of the enzymes with their respective substrates, mannitol-1-phosphate and sorbitol-6-phosphate.