This study was performed in two stages. The main purpose of the first experiment was to establish and to observe the bacterial plaque on the root surface in monkeys experimentally. Two adult male monkeys with normal dentition were employed and their 44 root surfaces of posterior teeth were used. A small plastic cover was fixed to the surgically denuded root surface. The cemental side of the cover had a groove to make room for plaque formation on the root surface. Surgical operation was made on

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... ach tooth 8, 7, 4, 3, 2 weeks and just before death of animals. The animals were fed with soft diet during the whole experimental period. Using light and electron microscopy, the bacterial plaque was found on the denuded cementum and the flora consisted of predominantly rod-shaped cells and some spirochetes 2 weeks or more after fixing the cover. By scanning electron microscopy, we also found the breakdown of collagen fibers which had remained on the surgically denuded cemental surface and calcified deposit on the cementum under the bacterial plaque. The purpose of the secend experiment was to observe whether the bacterial plaque can survive and artificial pocket can be left after removal of the cover or not. Two adult female monkeys were used. Each cover was fixed to each tooth for 4-6 weeks in the same way, and they were removed 6, 5, 4, 3, 2, 1 week and 3 days before the animals were killed. We found the bacterial population on the root surface rapidly decreased immediately after the removal of the cover. But we also found a few rod-shaped cells and spirochetes on the root surface in the pocket and the deep pocket remained especially on the palatal surface of the upper posterior teeth even 6 weeks after removal of the cover.