Using routinely processed, paraffin-embedded tissue specimens, osteoclast-like giant cells in giant cell tumour of bone (GCT), chondroblastoma, osteoblastoma and osteoblastic osteosarcoma were examined histochemically for osteoclast-specific enzymes tartrate-resistant acid phosphatase (TRAP) and carbonic anhydrase isoenzyme II (CA-II). Osteoclast-like giant cells and some mononuclear cells possessed TRAP activity. These were further classified with respect to CA-II immunoreactivity, i.e. cells with CA-II were seen in GCT and chondroblastoma, while those in osteoblastoma and osteoblastic osteosarcoma were negative for CA-II. All the cellular components in malignant fibrous histiocytoma and various extraosseous inflammatory lesions including malignant giant cells and macrophage polykaryons were negative for both TRAP and CA-II. These results indicate that osteoclast-like giant cells in GCT, chondroblastoma, osteoblastoma and osteoblastic osteosarcoma are all osteoclasts and generated by fusion of mononuclear cells with the same histochemical characteristics as osteoclast-like giant cells. The difference in CA-II immunoreactivity suggests the functional or maturational difference between osteoclast-like giant cells in GCT and chondroblastoma and those in osteoblastoma and osteosarcoma.