Structures resembling exostoses derived from periosteum and metaplastic nodules of cartilage and bone were induced in mouse legs by isotopic implants of minced skeletal muscle or heterotopic implants of nonlimb skeletal and cardiac muscle that does not regenerate. Liver mince implants produced no periosteal response. Control experiments demonstrated that, after excision of the muscle without the placing of any implant, no periosteal response or metaplastic bone occurred unless the periosteum was deliberately injured. Metaplastic nodules of cartilage and bone arose from the fibrous stroma that constituted the greater part of the implant when muscle mince regeneration was unsuccessful. The development of both the periosteal response and the metaplastic bony nodules is inversely related to the efficiency of phagocytosis by macrophages that remove the implanted debris and the concomitant degree of regeneration of new muscle. These phenomena, and, therefore, bone and cartilage formation, are strongly age related. The rapidity and extent of development of both abnormal bone and cartilage--less than 7 days--and its reproducibility may make this phenomenon a useful model for studies on the pathophysiology of bone and cartilage formation, particularly with regard to age.