In HCL, the addition of tartrate does not inactivate the enzyme acid phosphatase within the neoplastic cells – i.e. they are tartrate-resistant, a feature not seen in most other leukemias and lymphomas. With tartrate-resistant acid phosphatase (TRAP) stain, the hairy cells show red granular cytoplasmic staining. The TRAP activity is labile and the stain must be performed on air-dried smears soon after the slides are prepared. Nowadays, the TRAP cytochemistry has been largely replaced by TRAP immunohistochemistry (shown here). It must be remembered that TRAP immunostains are not as specific as TRAP cytochemistry and may stain other B-cell neoplasms. Image courtesy of: Jared Block, MD, Carolinas Pathology, Charlotte, NC; Sciplicity, LLC.