Tartrate-Resistant Acid Phosphatase (TRAcP), also known as type-5 acid phosphatase and purple acid phosphatase, is an iron-binding protein present in many cell types, including macrophages and osteoclasts. TRAcP is present in elevated levels in the spleen, as well as in monocytes of patients with Gaucher's disease, splenocytes and circulating lymphocytes of patients with hairy cell leukemia, the spleen of those with Hodgkin's disease, the sera of individuals undergoing active bone turnover, and in B-cell and T-cell leukemias and lymphomas. Maternal and embryonic tissues have also been found to have elevated levels of TRAcP in placental decidual cells, syncytiotrophoblasts, and some macrophages. Anti-TRAcP is the ideal choice for identifying hairy cell leukemia due to its high sensitivity and specificity.