The aim of the present investigation was to evaluate the caries-protective effect of two dentin adhesives on development of root caries after tooth-brushing abrasion in vitro. The root surfaces of fifty freshly extracted human molars were cleaned using polishing discs, thereby removing the cementum. The teeth were then coated with acid-resistant nail varnish, exposing two retangular windows of 6mm2 each. One window served as control, while the other window was tooth-brushed for different brushing cycles (group A1/2: 400x, group B1/2: 800x, group C1/2: 1200x, group D1/2: 1600x, group E1/2: 2000x) after dentin adhesive application. Respectively, 25 specimen were treated with one of the two dentin adhesives (1: Optibond Solo; 2: Prime & Bond NT) as recommended by the manufacturer. After brushing all specimen were demineralized with acidified gel (HEC, pH 4.8, 37 °C). From each specimen, three dentinal slabs were cut. The lesion depth was determined using a polarized light microscope. All unbrushed control specimen showed no lesions. In case of Optibond Solo lesions could be observed after 800 brushing cycles (A1: 0µm; B1: 31µm; C1: 12µm; D1: 13µm; E1: 37µm). The brushed specimen treated with Prime & Bond NT exhibited following lesion depths: A2: 0µm; B2: 36µm; C2: 55µm; D2: 52µm; E2: 57µm. Pairwise comparison showed significant differences between the two materials in group C, D, and E (ANOVA; p Keywords: dentin, dentin adhesive, root caries, demineralization, abrasion