Seventeen subjects with more than 3 × 105 mutans streptococci per millilitre of saliva completed this randomised, cross-over study. Four different chewing-gums, containing: (1) 70% xylitol, (2) 35% xylitol+ 35% sorbitol, (3) 17.5% xylitol+ 52.5% sorbitol, and (4) 70% sorbitol, were tested. The participants used 12 pieces of each gum per day for 25 days. During the four experimental periods, they wore a removable palatinal plate containing two demineralised enamel samples, and brushed their teeth with a non-fluoridated toothpaste. The results showed that an increased concentration of xylitol in the gum resulted in a lower number of mutans streptococci in both saliva and dental plaque, although the decreases were only significant in the saliva samples (p < 0.01). The pH drop in plaque, measured in vivo after a 1-min mouthrinse with a 10% sorbitol solution, was least pronounced after the 70% xylitol gum and most pronounced after the 70% sorbitol gum period (p < 0.01). No significant differences were found after a mouthrinse with a 10% sucrose solution. All demineralised enamel samples lost mineral during the experimental periods. However, the lesion depth as well as the mineral loss values, assessed microra-diographically, did not differ significantly between the four chewing-gums