Østergaard E. Evaluation of the antimicrobial effects of sodium benzoate and dichlorobenzyl alcohol against dental plaque microorganisms. An in vitro study. Acta Odontol Scand 1994;52:335–345. Oslo. ISSN 0001-6357.Evaluation of antimicrobial agents is based on in vivo and in vitro studies. The minimum inhibitory concentrations (MICs) of sodium benzoate and dichlorobenzyl alcohol to 115 strains of plaque microorganisms were determined by a broth-dilution method. Sodium benzoate did not inhibit growth of any gram-positive cocci (MIC>106,590µM). MICs forPorphyromonas gingivalisand two strains ofTreponema socranskiiwere 26,650µM. The MIC of dichlorobenzyl alcohol to the reference strain ofActinobacillus actinomycetemcomitanswas 723µM and toP. gingivalis, two strains ofT. socranskii, andCandida albicans1,446µM. MICs for other organisms were 2,892 to 5,784µM. Saliva samples from 10 volunteers, collected at various times after toothbrushing with a dentifrice containing 10% sodium benzoate and 0.3% dichlorobenzyl alcohol, were analyzed gas-chromatographically. Immediately after toothbrushing mean levels of sodium benzoate and dichlorobenzyl alcohol were 372,626µM and 7,529µM, respectively. After 5 min mean levels were 38,700µM and 734µM. In conclusion, the concentrations of both antimicrobials dropped rapidly during the first 30 min, but for 5–10 min they were high enough to inhibit growth of potential periodontal pathogens.