Transmission Electron Microscopy Of Dental Plaque After The Use Of Antimicrobial Mouthrinses A. Hombach*, E. Beetke, H. Jentsch (Dental School, University Of Rostock, Germany) In vivo studies demonstrated the inhibition of plaque growth by different antimicrobial mouthrinses. The effect on plaque microorganisms was analyzed mostly in vitro or by the determination of plaque vitality.   The aim of this study was to verify the suitability of transmission electron microscopy for the demonstration of effects of different antimicrobial mouthrinses on dental plaque properties in vivo. 21 adults aged 18 to 62 years participated randomized in a three days study. The subjects weared plastic splints with three pieces of bovine enamel in the molar area in the lower jaw. 0,12% chlorhexidine digluconate (group A), Meridol“ mouthrinse (group b) or 0,7% sodium chloride (group C ) were used for rinsing.   The rinsing procedure was executed with 10 ml mouthrinse for 30 seconds twice a day.   Toothbrushing and any other hygiene procedure were forbidden. 24, 48 and 72 hours after baseline each a piece of enamel with the plaque layer was removed and prepared for transmission electron microscopy. The dental plaque was separated from the enamel after fixation in glutaraldehyde and fixed. After dehydration in acetone the specimen were embedded in epoxy resin „Araldite“ (Fluka, Switzerland). Ultrathin slices were executed using an ultramicrotome (Leica, Austria). The dental plaque was contrasted with uranyle acetate and lead citrate for electron microscopy. The transmission electron microscope Zeiss, Germany, was used.   The morphology of plaque microorganisms was mostly destroyed after 24 hours in group A. No further plaque growth and no growth of rods was observed. After 48 hours the plaque microorganisms were completely destroyed. In group B an additional plaque growth as well as a growth of cocci and rods were observed in the course of these three days. The structure of plaque microorganisms was very seldom modified. No influence on plaque microorganisms was detected in group C.   This permits to conclude with some certainty that this model using plaque growth on bovine enamel and transmission electron microscopy could be a useful method of screening potential plaque inhibitory formulations in vivo.