Chlorhexidine digluconate for chemical plaque control was tested in different concentrations using a fractionated jet oral irrigator. The inhibition of plaque formation and the prevention of gingival inflammation were evaluated in a double-blind study. During a 10-day period of abstinence from any mechanical oral hygiene procedures, the pattern of plaque formation and gingivitis development under the influence of chemical plaque control was analyzed. As a positive control, one group rinsed twice daily with 30 ml of a 0.2% chlorhexidine solution while a group applying 600 ml of a placebo solution served as a negative control. Forty dental students and assistants with plaque-free dentitions and healthy gingival tissues were divided into four groups. After a 10-day period of no oral hygiene, a recovery period of 11 days with perfect oral hygiene was again instituted. This experiment was repeated three times so that a total of 10 concentrations in the irrigator, the control rinsing and the placebo control could be evaluated. Daily application of 600 ml of a 0.001% (6 mg), 0.0033% (20 mg), 0.005% (30 mg), 0.01% (60 mg), 0.02% (120 mg), 0.05% (300 mg) and 0.1% (600 mg) and 400 ml of a 0.015% (60 mg), twice 400 ml of a 0.015% (120 mg) and 400 ml of a 0.02% (80 mg) solution of chlorhexidine was tested. At the start of each experimental period (day 0), after 3, 7 and 10 days and 11 days following reassuming oral hygiene procedures, the plaque accumulations were determined using the Plaque Index System (Silness & Löe 1964) and the development of gingivitis was evaluated according to the criteria of the Gingival Index System (Löe & Silness 1963). The results suggested that one daily irrigator application of 400 ml of a 0.02% chlorhexidine solution was the optimal and lowest concentration and dose to be used for complete inhibition of dental plaque.