The tissue reaction following apical plugging of the root canal with dentin chips in monkey teeth subjected to pulpectomy was studied. The reaction was characterized by formation of a cementum-like tissue onto the chips, often walling off the root canal as well as accessory and lateral canals. Twenty-two of twenty-four teeth were rated as successful 95 days after pulpectomy. In the control groups, in which apical plugging of the canal with dentin chips was not attempted, ten of twenty-one teeth were rated as successful at this observation time. In six of the ten successfully treated teeth in these groups a dentin chip plug had been made, although care had been taken to avoid this. It is suggested that a plug of dentin chips is well tolerated by the tissues and may present an effective barrier in the apical part of the root canal, facilitating the accomplishment of a well-condensed, tightly sealing root filling.