Aim: The aim of this stydy was to assess whether supplementation of Lactobacillus reuteri could have an impact on the oral microbiota. Material and Methods: Twent-three healthy people aged 29 to 63 years were included. A randomised double-blind placebo-controlled study was executed consisting of 12 persons in the test group which was given the study product twice a day for twelve weeks containing L. reuteri (an equal mix of ATCC 55730 and PTA 5289 at a total of 2x108 CFU/dose) and the control group of 11 persons having corresponding placebo. Pre and post of study period plaque index and oral health status (gingivitis, probing pocket depth, bleeding on probing) were measured together with sampling of supra-, subgingival plaque and saliva collection. Microbiological analysis was done by using checkerboard DNA-DNA hybridization technique and selective culturing for lactobacilli determination. Four weeks after the last intake of the product reassessments of plaque and saliva was performed. Results: No difference in general oral health could be observed between the groups after L. reuteri supplementation. Plaque index increased significantly in the control group after twelve weeks (p= 0.023). A significant increase in total Lactobacillus counts in saliva occurred in both groups (p<0.05). The probiotic intervention resulted in a significant increase of L. reuteri (p=0.008) corresponding to 13.8% of the total lactobacilli couont. A distribution ratio of 1:4 (ATCC 55730/ATCC PTA 5289) between the two installed L. reuteri strains in saliva was noticed. Termination of intervention resulted in a wash out of L. reuteri. A significant increase was found for most bacterial species in both groups and both in supra- and subgingival plaque during the test period. There was no significant difference detected for any of the bacterial species between the groups neither in plaque location. The ratio between "bad/good" supragingival bacteria decreased for the test group, however, not significant. The corresponding ratio for subgingival bacteria decreased significantly for both groups (p= 0.05)with no significant difference between the groups. Conclusions: The supplementation of L. reuteri did not affect general oral health. Presence of L. reuteri in saliva is only temporary and washed out after termination of intervention. Microbiologically no significant effect of the probiosis was observed. It can not be concluded whether L. reuteri was established in the plaque of the test group or not.