mechanism of osteogenesis in the repair of fractures, but practical therapeutic applications are still very few. Immobilisation of the fracture is the only favourable element known with fair certainty. It is notable that, in man, no physical agent or drug convincingly shortens the time or increases the frequency of bony union in diaphysial fractures. Many biological factors play a part in callus formation, but the ones which might usefully be influenced are still unknown. Recently, the main body of experimental work has been devoted to the study of metabolic, vascular, cellular, hormonal, mechanical and electrical factors. The present study was undertaken to assess the efficiency of autografts of cortical and cancellous bone, and of free periosteal grafts, in recent diaphysial fractures. Simple diaphysial fractures tend to unite spontaneously, provided that they are adequately immobilised. This is so in man, and even more so in animals. Consequently it is somewhat difficult in the study of callus formation to demonstrate any favourable factor. Therefore we began by creating an experimental system, incorporating a standardised severe fracture in animals. MATERIALS AND METHODS Fully grown rabbits were used, all of the same strain (fauves de Bourgogne) and aged fifteen to eighteen weeks. The severity of the fracture was ensured by excising the periosteum widely, either in single fractures, or in triple-level fractures. This is fairly easy in the rabbit in which the tibia and fibula are fused into one bone. The triple-level fractures consolidated very slowly, and the response to different kinds of grafts was studied in this type of standardised lesion. Operation-All the fractures were made in the tibial shaft with a circular saw driven by a dental drill motor. They were stabilised by a medullary pin two millimetres in diameter and the limb was immobilised in plaster. This was made in two separate pieces, a knee-and-thigh piece and a foot-and-ankle piece, firmly linked together by two lateral splints. Thus the middle of the leg was not covered by the plaster and weekly radiographs of the callus could be taken under general anaesthesia without removing the plaster. The knee and ankle were each immobilised at a right angle. Penicillin injections (500,000 units) were given daily in the first week after operation. All the fractures were managed as described above. Seven different types of fracture were studied: Group 1 : Singlefractures (periosteum not excised)-The bone was exposed subperiosteally by a longitudinal incision through all soft tissues on the anterior aspect of the leg. The periosteum was completely detached from the circumference of the bone for a length of about half a centimetre and the bone was divided. At the end of the operation the periosteal incision was not sutured. Group 2 : Singlefractures with excision ofperiosteum-The bone was exposed extraperiosteally * Former Scholar at the Nuffield Orthopaedic Centre, Oxford.