1. Penetrating defects were cut in the femora of twenty-five albino rats. In fifteen of the animals the defects in the right legs were protected with cellulose-acetate shields while those in the left legs were unprotected and allowed to heal as controls. In the remaining ten animals the defects in both legs were protected with shields made of homogenous organic bone.

2. New bone was found to proliferate into the concavity of the shields in most of the animals and this protruded beyond the contour of the femur. The development of the protuberance appeared to depend upon the degree to which the shield was adapted to the femoral surface.

3. The cellulose-acetate shield was not removed by the host, but the homogenous organic bone was actively resorbed; multinucleated giant cells were associated with this process.

4. There are indications that the maintenance of the protuberance is dependent upon the continued presence of the shield. Exostoses protected by intact cellulose-acetate shields have been recognised up to eighteen months after operation.

5. The function of the shield in the formation of the bony protuberance is thought to be two-fold, in that it protects the haematoma from invasion by non-osteogenic extra-skeletal connective tissue, and that it governs the size of the haematoma and prevents its distortion by the pressure of the overlying soft tissue.