An attempt was made to assess the local chronic inflammatory response in patients with failed orthopaedic implant that is clinically associated with osteolysis, bone and bone marrow necrosis. The main objective was to analyse the heterogeneity of the macrophage functional subsets in the bone-implant interface membrane and to evaluate their possible role in the development of an erosive inflammatory lesion within the bone. Immunohistology was performed on 21 specimens of the bone-implant interface obtained from 17 patients during revision arthroplasty, and synovial membranes from rheumatoid (RA, n=4), and osteoarthritis (OA, n=4) patients. Three well-characterized monoclonal antibodies (MAb) recognizing antigenic determinants on specific functional subsets of macrophages (Mphi) were used. RFD1 (interdigitating reticulum cells/antigen presenting cells, (APC), RFD7 (mature phagocytic macrophages), and RFD9 epithelioid cells and foreign body giant cells (FBGC). RFD1 was expressed on a variable number of perivascular and synovial lining Mphi in both RA and OA synovia, at a frequency of 25%-40%. In cases with total joint replacements, the interface showed a marked increase in the expression of RFD1 (20%-90%). A considerably greater percentage of RFD1 positive Mphi and FBGC was noted in the interfaces from cases with a high level of detectable metal particulate wear debris (mean 80%, range 60%-90%) than in cases with polyethylene wear debris (mean 30%, range 0%-50%), p 0.0001. RFD7 labelled most tissue Mphi in each group. Immunoreactivity for RFD9 was restricted to FBGC in all cases analysed. The finding of elevated expression of RFD1 on metal-containing Mphi and FBGC in the bone-implant interface suggests an increase in antigen-presenting phenotype and indicates that metal particles have more impact in the induction of immune-mediated responses. Such responses are characterized by sustained cellular hyperreactivity and phenotypic changes in Mphi subsets.