Aim Osteoclasts are multinucleated cells which are specialised to carry out lacunar bone resorption. Osteoclasts form part of the mononuclear phagocyte system, and immunophenotypic criteria for distinction from macrophage polykaryons include expression of CD51 (vitronectin receptor) and absence of HLA-DR and CD14. Methods The expression of CD14, CD163, HLA-DR and CD51 in formalin-fixed paraffin-embedded sections of normal bone and neoplastic and non-neoplastic lesions of bone and soft tissue known to contain osteoclasts and macrophage polykaryons respectively was assessed immunohistochemically; the immunophenotype of osteoclast-like giant cells in a wide range of giant cell-containing bone lesions was similarly assessed. Results Both osteoclasts and macrophage polykaryons were found to express CD51. Macrophage polykaryons, but not osteoclasts, expressed CD14 and HLA-DR. CD51+/CD14−/HLA-DR-/CD163– giant cells were noted in all giant-cell lesions of bone, including giant cell tumour of bone, aneurysmal bone cyst, non-ossifying fibroma, chondroblastoma, telangiectatic osteosarcoma, chondromyxoid fibroma, Langerhans cell histiocytosis and brown tumour. Conclusion Our findings indicate that CD51 expression alone is not sufficient for immunocytochemical identification of osteoclasts, which do not express the macrophage-associated antigens CD14 and HLA-DR. Giant cells in most giant cell-rich lesions of bone have an osteoclast phenotype, suggesting that they are formed from mononuclear phagocyte osteoclast precursors.