This study describes an in vivo inhibition of dental plaque growth after peroxidase-generated hypoiodite (OI-) mouth-washes. After giving up all other usual hygiene procedures nine healthy volunteers washed their mouth using 10 ml of the mouth-wash [H2O2 (0.005%), KI (50 mM) and lactoperoxidase (0.04%)] three times a day for 1 minute for 3 days. The initial oxidation power of this mixture represented 430 +/- 11 microM oxidised cysteine (n = 6), dropping down to 87 +/- 6 microM after the solution was spat out (n = 5). A saline solution served as a negative control, and a 0.2% chlorhexidine digluconate solution as a positive control. Proximal dental plaque between mandibular canine and lateral incisor (left and right) was collected after 3 days using standardized sterile toothpicks, then analysed for ATP and protein content. ATP concentrations dropped to 49% of the control values after OI- rinsing, and to 9% after chlorhexidine rinsing while the protein content dropped to 48% for OI- versus 31% for chlorhexidine. However, when considering the ATP content per protein micrograms, only the decrease to 6% of the initial value in the chlorhexidine testing was significant while the drop to 81% for the OI- testings was not significant. This study points out a negative effect of OI- on plaque growth in vivo.