Osteoclastogenesis is a key event of the cellular reaction in prosthetic loosening. Immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) were used to study the localization and expression of receptor activator of nuclear factor kappa B ligand (RANKL), a potent factor for osteoclastogenesis in the membranous tissue formed around loosened prosthetic joint implants. RANKL was identified in a wide variety of cells appearing in this membranous tissue. At least three types of RANKL-positive cells were identified, including prolyl 4-hydroxylase (PH)-positive fibroblast lineage cells, CD68 cells, and tartrate-resistant acid phosphatase (TRAP)-positive mononuclear and multi-nucleated macrophage lineage cells. Tumor necrosis factor (TNF)-alpha-converting enzyme (TACE) was colocalized with RANKL in these cells, suggesting the in-situ release of this factor. RT-PCR confirmed the actual expression of the RANKL and TACE genes in the tissues around the loosened implant. These observational findings indicate the possible synthesis of RANKL by fibroblast and macrophage lineage cells, and suggest the in-situ involvement of RANKL in both osteoclastogenesis and osteoclastic bone resorptive events occurring in prosthetic joint loosening.