The authors investigated changes in the location and number of osteoclasts and their precursors during the development of articular lesions in type II collagen-induced arthritis in mice using tartrate-resistant acid phosphatase (TRAP) staining. The limb joints were examined at 6 to 15 weeks after the second immunization. The number of TRAP-positive cells increased as the articular lesions progressed. TRAP-positive macrophage-like cells were found in the hyperplastic synovial tissue and bone marrow stroma in the early stage. In the advanced stage, in addition to many TRAP-positive osteoclasts on the bone surface, TRAP-positive macrophage-like cells were observed in the pannus apart from the bone surface in the pannus-joint junctions. The above mentioned TRAP-positive macrophage-like cells are considered to be osteoclast precursors.