Implant materials, for examples, plaster of Paris, Bone cement and ceramics which are used experimentally to regain shape and function, act as a scaffold for bone growth and contribute to the healing processes at bone defect developed by trauma, infection and other causes. Therefore, the effect of plastei of Paris on the bone defects of adult rabbits may be an interesting subject, particularly in the orthopedic field. The authors carried out an experiment to study the effect of plaster of Paris on the bone defect, using 30 adult rabbits weighing fmm 3000-3500 gm, which were divided into 3 main gmups. (Each group was composed of 10 adult rabbits.). In Group 1, which was the control group, made a defect at the proximal metaphysis of the tibia, 6 mm×6 mm×6 mm in size, using a drill bit and filled by hematoma spontaneously. In Group II and Group IU, a defect was also made by the same method as in Group I and a piece of Gelfoam or plaster of Paris was inserted into the bone defect respectively. Then we studied the healing processes of the bone defect by sacrifying animals of each group at 6th and 12 th week after operation. The results of this experiment were as follows. 1. New bone formarion in the bone defect was poor in group I and Gmup Il, but it was good in Group III. 2. The plaster of Paris, filling into the defect, was absorbed and removed from the site of implantation continuously and stimulated the new bone formation. 3. The Gelfoam in Group II was more rapidly absorbed than plaster of Paris and didn't stimulate the new bone formation.