Naringin is a flavonoid that is commonly found in grapefruits. The objective of this study was to evaluate the effects of naringin on the growth of periodontal pathogens such as A. actinomycetemcomitans and P. gingivalis in vitro. For comparison, the effects of naringin on several oral microbes were also studied. Different concentrations of naringin solution were added to calibrated suspensions of A. actinomycetemcomitans and P. gingivalis. All the suspensions were incubated for 3, 6 and 24 h in an anaerobic chamber at 37 degrees C. At each time point, selected dilutions from each culture broth were plated on blood agar plates. Colonies recovered on blood agar were visually counted on days 3 and 5, respectively. A. actinomycetemcomitans showed a significant decrease (p < 0.05) in viable counts after 3 h when naringin was added at baseline. P. gingivalis also showed a marked growth reduction in the presence of naringin, and no colony forming units could be observed after 24 h. Naringin also had an inhibitory effect against all bacteria and yeasts tested. The results suggest that naringin possesses significant antimicrobial properties on periodontal pathogens in vitro. It also has an inhibitory effect on some common oral microorganisms in low concentrations.