A metastabilized chlorous acid/chlorine dioxide (MECA) formulation was used as a mouthwash in a group of 18 volunteers aged 20-27. Its effect on developing plaque and salivary bacterial count was tested. The trial was carried out over 33 days during which each subject used three different formulations of mouthwash: a high concentration (0.16% sodium chloride in an activating system), a low concentration (0.04% sodium chloride, comparably activated) and a placebo mouthwash (activating system alone). Each participant used each of the three formulations as the only means of oral hygiene for 5-day periods. Each experimental period was separated by 9 days during which the participants returned to their regular oral hygiene habits. The high concentration and low concentration groups showed a 34.5 and 13.5% reduction of dental plaque scores, respectively, compared with the placebo control group. This effect on the plaque index scores was not accompanied by any significant change in the number of salivary bacteria.