Histological, histochemical and ultrastructural studies were done on soft tissue surrounding alloarthroplastic joints. In 38 cases a prosthesis of the hip joint and in 2 cases of the knee had to be exchanged and replaced. In most of the cases the reoperation became necessary because the anchoring of the prosthetic parts in the bone loosened. Up to 18 months after the first operation infection was responsible for the malfunctioning in some cases. Other complications were luxation and material faults. The morphological changes are determined by the tissue reaction to the different alloplastic materials used and by the time interval they remained in the organism. The large polymerized acrylic cement particles are phagozytosed by multinucleated foreign body giant cells. About 12 months following the implantation of the artificial joints small double refractile particles appear and evoke characteristic morphological changes. The particles are abraded by the continuous friction of the moving alloplastic or metallic surfaces of the prostheses. Usually they are phagozytosed by histiocytes, which form large granulomas and undergo degenerative changes as is indicated by the ultrastructural and histochemical findings. These alterations are more pronounced and occur sooner in prosthesis with parts (rotation ball or cup.) fabricated by polyester than in those made by polyethylene. The abraded particles not only are transported to the inguinal lymphnodes, but also to the tissue between prostheses and bone, where they induce the same morphological changes as in the capsule. Hence the fibrous membrane separating bone and prostheses increases in width, and the spongy bone is partially destroyed by the proliferating histiocytes. It is assumed that by impairing the anchoring this foreign body reaction to the abraded alloplastic particles is the leading cause of the loosening of this kind of artificial joints.