Tartrate-resistant acid phosphatase (TRAP) is a basic iron-binding protein with high activity towards phosphoproteins, ATP and 4-nitrophenyl phosphatase. It is detected in human alveolar macrophages, osteoclasts, spleen and liver. Its expression is increased in spleen and monocytes of patients with Gaucher’s disease; spleenocytes and circulating leukocytes of patients with hairy cell leukemia; spleen of patients with Hodgkin’s lymphoma and in the sera of patients undergoing active bone turnover.