The ability of Lactobacillus salivarius TI 2711 (LS 1) to displace periodontopathic bacteria, like Porphyromonas gingivalis and Prevotella intermedia, was studied using humanvolunteers. LS 1 was one thousandfold more susceptible to lactic acid than Lactobacillus acidophilus, a representative acid-resistant Lactobacillus strain frequently found at the sites of caries, when these bacteria were exposed to 50 mM of lactic acid. In an in vitro system, LS 1 completely killed P. gingivalis within 24 hours when these bacteria were cultured together. In a clinical study, 57 subjects took tablets containing 2×10⁷ CFU or more of LS 1 daily for 4 or 8 weeks. The number of black-pigmented anaerobic rods, which includes most periodontopathic bacteria, in the saliva decreased to one-twentieth of the initial value after 4 weeks, whereas the numbers of whole bacteria, Streptococcus mutans and lactobacilli did not change. While the saliva pH was widely distributed (ranging from 5. 4 to 8. 5) before LS 1 treatment, it converged to within a neutral range of around 7. 3 after treatment. Thus, the possibility that LS 1 accelerates caries formation by lowering the pH in the oral cavity was excluded. These findings suggest that LS 1 may be, potentially useful probiotic agent against periodontopathic bacteria. J Jpn Soc Periodontol, 45 : 105-112, 2003.