The effects of mutanase (α-1,3 glucan 3-glucanohydrolase, free of 1,6 activity produced by Trichoderma harzianum OMZ 779) and dextranase (α-1,6 glucan 6-glucanohydrolase obtained from Penicillium funiculosum) on plaque formation and caries incidence were investigated in two groups of 36 rats each. The first group was kept in relative gnotobiosis with Streptococcus mutans OMZ 176 E, the second group harboured its indigenous flora. During the entire 28-day experimental period the animals were housed in a feeding machine and fed 18 meals per day. For the first three experimental days both groups received diet 2,000 (56% sucrose). Thereafter each of the two groups was distributed among the following treatments: O: diet 2,000; M: diet 2,000 containing mutanase (100 MU/ g); D: diet 2,000 containing dextranase (150 DU/g); M+D: diet 2,000 containing both enzymes (100 MU and 150 DU/g). Blood for white cell counts was collected prior to sacrifice by heart puncture. Extent of plaque on the smooth surfaces of the upper molars was scored under a dissection microscope after staining with erythrosin. Fissure caries was assessed on serial sections of the first and second lower molars.