Objective: Evaluate the cellular mechanisms responsible for the aseptic loosening of total hip replacements. Methods: Twenty periprosthetic membranes were collected during revision procedures at the Department of Orthopedics, Hacettepe University, Ankara, Turkey, during 1998 to 1999. Arthroplasties with polyethylene components and without polyethylene components were examined under light microscopy and immunohistochemically using the indirect immunoperoxidase technique into 2 different groups. Results: Immunohistochemical analysis with monoclonal antibodies reactive to certain cell surface antigens (CD45, CD44, CD98, CD31, CD26, CD71) revealed different morphological characteristics regarding the adjacent fibrosis, leukocyte activation, and immunologic response. Conclusion: These findings suggested the role of different cellular mechanisms in each group.