The purpose of the present study was to find out to what extent subgingival plaque formation may be prevented by toothbrushing. The experiment was carried out on a total of 28 molars in four monkeys. On day 0 all supra‐ and subgingival deposits were removed, and during the following year, the teeth on the left side were carefully brushed three times a week, employing the Bass' method. The right side was kept as unbrushed control. The histologic sections, which were cut in a bucco‐lingual direction, showed that subgingival plaque almost invariably had developed on the unbrushed teeth. Although all of the brushed teeth were free of subgingival plaque, a mild to moderate cellular infiltration prevailed for some distance below the gingival margin. These inflammatory reactions were assumed to have been induced by the bristles of the toothbrush, which were shown to penetrate as far as 0.9 mm below the gingival margin, when employing the Bass method. It was concluded that subgingival plaque formation can be prevented in areas accessible to the toothbrush.