The aim of the present study was to determine the nature of the cells associated with the formation of reparative acellular extrinsic fiber cementum (AEFC). Calcifying collagen membranes, hydroxyapatite particles and/or non-resorbable ePTFE membranes were implanted in lesions made in the periodontium of the rat mandibular incisor. The incisor was prevented from erupting further, and the animals were killed after 1-8 wk. In the first week, next to cells with a fibroblastic phenotype, epithelial cells (ECs) migrating out of the reduced enamel epithelium were among the cells colonizing the wounds. From 2 wk on, the ECs showed regressive changes and disappeared. Along mineralized implant surfaces, a basophilic layer enriched in osteopontin (but without detectable amelogenin) was deposited. After 3 wk (when ECs were no longer present) the healing tissue transformed into a well-organized PDL-like tissue, and AEFC started to develop. Along the non-mineralizing ePTFE membranes, AEFC did not form except in regions where small calcified bodies were present. It is concluded that reparative AEFC layers are formed only along calcified surfaces. The cells associated with this reparative activity are periodontal ligament cells with a fibroblastic phenotype.