We studied the efficiency of various bone grafts in the assessment of new bone formation by tetracycline labeling in sixty rabbits. The rabbits were divided into four different groups. Vascularized tibial cortical segment(Group I), devascularized free cortical graft without intramedullary reaming (Group II-A), devascularized cortical graft with intramedullary reaming(Group II-B) and autoclaved free cortical graft(Group III) were evaluated by quantitative analysis of new bone formation after double labeling with tetracycline. Group I was found to produce most abundant periosteal new bone formation, and followed by Group II-A. Group II-B and Group IU showed approximately same amount of new bone formation. While endosteal new bone formation was predominant in Group II-A, periosteal new bone formation was more predominant in Group II-B. Intramedullary reaming seemed to inhibit endosteal bone formation in devascularized free cortical graft by interfering with the ingrowth of the vascular channels and granulation tissue through the medullary cavity.