0 v ) V ) n 100 ng 50 25 12 6 3 Mouse mAb 9 C 5

Tartrate-resistant acid phosphatase (TRAcP) has been an indispensible marker for hairy cell eukemia (HCL) for over two decades. However, the traditional TRAcP cytochemical stain cannot be performed effectively on sections of paraffin-embedded tissues that are important resources for histopathologic evaluation in diagnosis and treatment of HCL. Wide variation in expression of TRAcP activity by hairy cells (HCsl within and among patients is an interesting biologic phenomenon that has not been explained and can cause some diagnostic uncertainty as well. To solve the problem of staining TRAcP in paraffin sections and to begin to address the questions of variable TRAcP expression in HCL, we developed a monoclonal antibody to TRAcP, 9C5, for immunohistochemical identification of HCs. In smears of blood and bone marrow, immunocytochemistry of TRAcP using 9C5 was as specific but slightly less sensitive than direct cytochemical staining of enzymatic activity. In paraffin sections