In vitro studies on abrasion caused by hand and automatic toothbrushing mo tions and by nylon bristles, natural bristles and rubber-tip stimulators were performed. ‘Comparisons were made on a weight-loss and a relative volume-loss basis. In comparison with the shortstroke, reciprocating-action automatic toothbrush, under simulated conditions, arcuate-motion automatic toothbrushing produced 160 per cent more abrasion; hand scrub brush motion produced 80 per cent more abrasion, and occlusalward hand toothbrushing produced 50 per cent less abrasion. On ivory and acrylic resin, natural bristles caused about twice as much abrasion as nylon bristles. Bristle buckling and matting were found to be important factors in the abrasion charac teristics of toothbrushes.