Fractures were created in the femur of dogs by dividing the midshaft with an airdriven saw. Immobilization was achieved with a four-hole compression plate. No methylmethacrylate supplementation was used in the control groups. In the three experimental groups methylmethacrylate was added to the internal fixation by placing it on the endosteal, periosteal, and both surfaces of the bone. Healing of the fractures uniformly failed to occur when methylmethacrylate was apposed to both periosteal and endosteal regions. Histologic, fluorescent, and autoradiographic studies indicated that the bone surface in contact with methylmethacrylate was necrotic and did not incorporate tetracycline or Ca-45. New bone occurred only on the surface which was not in contact with methylmethacrylate.