AIM: This study was designed to evaluate whether the oral administration of lactobacilli could change the bacterial population in supra/subgingival plaque. MATERIAL AND METHODS: Sixty-six healthy volunteers without severe periodontitis were randomized into two groups to receive lactobacilli or placebo for 8 weeks (8W): the test group (n=34) received 2.01 x 10(9) CFU/day of Lactobacillus salivarius WB21 and xylitol in tablets; the control group (n=32) received placebo with xylitol. Supra/subgingival plaque samples were collected at the baseline and after 4 weeks (4W) and 8W. The bacterial amounts in plaque samples were analysed by quantitative real-time polymerase chain reaction. RESULTS: The numerical sum of five selected periodontopathic bacteria in the test group was decreased significantly in subgingival plaque at 4W [odds ratio (OR)=3.13, 95% confidence intervals (CI)=1.28-7.65, p=0.012]. Multivariate analysis showed that significantly higher odds were obtained for the reduction of Tannerella forsythia in subgingival plaque of the test group at both 4W (OR=6.69, 95% CI=2.51-17.9, p<0.001) and 8W (OR=3.67, 95% CI=1.45-9.26, p=0.006). CONCLUSION: Oral administration of probiotic lactobacilli reduced the numerical sum of five selected periodontopathic bacteria and could contribute to the beneficial effects on periodontal conditions.