Tatrate resistant acid phosphatase (TRAP) has been widely used as histochemical marker to identify osteoclast in studies of bone metabolism. However, the value of TRAP as an osteoclast marker is still in discussion. Authors isolated and characterized the cells from synovium of 6 patients of sero-positive rheumatoid arthritis and 4 patients of osteoarthritis, and observed the activity of acid phosphatase (AP) and TRAP. The activity of TRAP was negative in cell cultures in early phase, but the activity of TRAP was increased with time, and at one week the activity of TRAP was as strong as that of AP. If the cultured tissue contained bone, there were observed TRAP positive mononuclear cells and giant cells even in early phase of cultures (1 day, 3 day), and the phenotype of these cells were same as that of osteoclasts and osteoclast precursors by immunocytochemistry. In conclusion, the activity of TRAP was positive in cultured macrophage. TRAP is not a specific marker for osteoclast, and its use for the identification of osteoclast seems meaningful only in the early stage of cell culture.