The fluid pressure and shear forces generated by the high frequency bristle motion of the Sonicare sonic toothbrush remove adherent colonies of cultured bacteria from model dental surfaces in vitro. These dynamic fluid effects can remove bacteria in vitro even at distances up to 4 mm beyond the tips of the bristles. To evaluate the efficacy of the Sonicare in removing actual human plaque deposits formed in vivo, an intraoral model was developed. Enamel sections were obtained from extracted human teeth and mounted on acrylic resin palatal prostheses, worn by two volunteers. Six enamel sections were arranged as three pairs at different locations on the prosthesis, and plaque was allowed to form overnight (approximately 16 h). The sections were removed, placed in phosphate-buffered saline, and exposed in vitro to the sonic toothbrush for 5, 10 or 15 seconds. The bristle tips were maintained at distances of 2 or 3 mm from the enamel surface. As a comparison, sections were also exposed to another electric toothbrush (Interplak) for 10 seconds using a distance of 3 mm between the bristles and the enamel. Following exposure to the toothbrushes, residual bacteria were removed from the sections by ultrasonication for 15 seconds, and total viable cell counts determined by serial dilution on blood agar plates. One section from each pair was used to measure total (baseline) microbial accumulation. At a distance of 3 mm between bristles and enamel, the sections exposed to Sonicare demonstrated significant (p < 0.001) plaque removal of 56-78% relative to non-treated controls. In contrast, the control electric brush did not demonstrate removal of plaque bacteria after 10 seconds exposure. These quantitative results were visually confirmed by scanning electron microscopy. The findings demonstrate that the fluid dynamic activity generated by the sonic vibrations of the Sonicare toothbrush removed microbial plaque formed in vivo, even at a distance of 3 mm beyond its bristle tips.