The premolar teeth were extracted from eight dogs, and cells from their periodontal ligaments and from attached gingiva were cultured in vitro. The lateral incisors were extracted later, the periodontal ligaments and pulps removed and the root canals filled. The roots were demineralized for 48 h in 0.5M EDTA, pH 7.4, and then divided into three groups. Cultured gingival cells were attached to those in the first group, psriodontal ligament cells to those in the second and no cells to those in the third which acted as controls. The roots were transplanted to holes drilled in the edentulous premolar region of the mandibular alveolar process and completely covered by a gingival flap, the roots and cells always being returned to the same dog from which they were removed. Because of infection, only 20 of the roots were recovered and, of these, only 4 bore periodontal ligament cells. All of the roots exhibited ankylosis and resorption. Only the roots bearing cultured periodontal ligament cells were associated with fragments of new fibrous tissue which was identified as periodontal ligament on the basis of the orientation of its fibres, the presence of Sharpey's fibres in bone and tooth, and cells located in positions normally occupied by osteoblasts and cementoblasts.