AbstractStudies on periodontal repair to denuded root surfaces have suggested that initial clot adhesion to the root surface may be important for the nature of subsequent healing. To study this hypothesis, circumferential periodontal defects, approximately 5 mm in vertical dimensions, were surgically created and immediately treated around the mandibular premolars in 4 beagle dogs. Prior to wound closure, the root surfaces were treated with either the anticoagulant heparin or with saline. Tissue blocks were obtained at sacrifice 4 weeks after surgery. Histometric analysis showed that connective tissue repair to the root surface averaged 50% of the defect height for heparin‐treated teeth as compared to 95% for saline‐treated teeth. Junctional epithelium amounted to an average of 33% of the defect height in heparin‐treated teeth in contrast to 5% following saline treatment. It can be concluded that heparin treatment of the root surface compromises connective tissue repair, confirming clot adhesion as one prerequisite for connective tissue repair of periodontal de