Acid phosphatase activity is indicated by a red granular precipitate and is demonstrable in most cells of the hemopoietic system. Intense activity is present in osteoclasts and some macrophages. Moderate staining is seen in plasma cells, megakaryocytes, and monocytes. Weak reactions can be observed in neutrophils, bands, metamyelocytes, myelocytes, and promyelocytes. Very little acid phosphatase activity is present in normal lymphocytes and erythroblasts. Increased acid phosphatase activity is observed in abnormal mononuclear cells of patients with hairy-cell leukemia, lymphocytes from patients with macroglobulinemia, atypical lymphocytes from infectious mononucleosis, and lymphoblasts from patients with T-cell leukemia. Tartarate-resistant acid phosphatase reaction diffusely prominent in the cytoplasm of neoplastic cells is highly characteristic of hairy-cell leukemia.

Acid phosphatase can hydrolyze a variety of hydroxynaphthoic anilides, releasing insoluble naphthols that efficiently couple diazonium salts at acidic pH. The resultant product forms a colored precipitate and gives good microscopic enzyme localization.