Ideal endosseous implant placement involves a congruent bony housing in close apposition to the implant surface. Clinical situations are encountered, however, in which the entire implant surface cannot be in close apposition to bone. In these instances, bone grafting materials are generally used to regenerate bone around the implant. In this study, a biologically active bone differentiation factor, recombinant human bone morphogenetic protein-2 (rhRMP-2), was used with two different carriers to regenerate bone around implants in standardized critical-sized defects in the canine mandible. Half of the sites had a nonresorbable membrane placed over the defect. Longitudinal standardized radiographs were obtained to assess the amount of bone regeneration on the mesial and distal of the implants after 4 and 12 weeks of healing. Ninety-six implants were placed in 12 foxhounds. Bone fill was determined by linear measurement of bone on the radiographs, and changes in bond density were evaluated by com puter-assisted densitometric image analysis of discrete areas adjacent to the implant. After 4 weeks of healing, nonmembrane sites had significantly greater bone height that membrane-protected sites. Following 12 weeks of healing, sites treated with rhBMP-2 had significantly greater bone formation than untreated sites. Sites treated with rhBMP-2, whetehr with or without a membrane, had less bone fill than sites with rhBMP-2. At 12 weeks, sites with a m embrane resulted in significantly mroe gain in bone density than sites without a membrane. Furthermore, sites treated with a collagen carrier resulted in greater gains in bone density than sites treated with a polylactide/glycolide carrier. The results from this study demonstrate by radiographic evidence that new bone formation in cirtical-sized defects around implants is dependent on time after defect treatment, the type of carrier used, the use of a barrier membrane, and the presence of rhBMP-2. In addition, these findings suggest that rhBMP-2, a bone differentiation factor, can significantly stimulate bone formation around endosseous dental implants.