A 9-month double-blind controlled clinical study was conducted on adult subjects using either Listerine antiseptic, its vehicle control, or a water control in order to determine the efficacy of the antiseptic mouthrinse in inhibiting the development of plaque and gingivitis. Following screening examinations for minimal entry levels of plaque and gingivitis, all subjects received a complete prophylaxis. Subjects then continued their usual oral hygiene habits for a 3-week normalization period and were examined for soft tissue abnormalities and baseline measurements of plaque, gingivitis, and tooth stain. 2 additional prophylaxes were then performed, followed by a second baseline gingival examination. Zero plaque was re-established by rubber cup polishing and twice daily rinsing was begun. Soft tissue, plaque, gingivitis, and extrinsic tooth stain were evaluated after 1, 3, 6 and 9 months of rinsing with the randomly assigned mouthrinses. Results demonstrated that Listerine antiseptic significantly reduced the development of plaque at 1, 3, 6 and 9 months and the development of gingivitis at 9 months, as compared to its vehicle control or water control.