Differential characterization of the "reticulum cell" in lymphoreticular neoplasms. Am J Clin Pathol. 64: 171-179, 1975. The term "reticulum cell" is confusing, having been applied to the cells involved in many hematopoietic neoplasms, such as reticulum-cell sarcoma, histiocytic medullary reticulosis, leukemic reticuloendotheliosis, and monocytic or histiocytic leukemias. In histologic sections, even the cells from poorly differentiated extramedullary lesions of chloroma or myeloblastic leukemia have been called "reticulum cells."A combined morphologic and cytochemical approach has been used to study "reticulum cells"in smears and tissue sections of neoplasms involving "histiocytes" or "reticulum cells."The cytochemical markers are: chloracetate esterase for neutrophilic granulocytes; nonspecific esterase and fluoride-resistant esterase for monocytes and histiocytes (phagocytes); tartrate-resistant acid phosphatase for the reticulum cells of leukemic reticuloendotheliosis; pyronin for the lymphatic reticulum cells (germinal center cells). The morphology of these cells is very well appreciated in smears, and the locations of these marked cells in tissue sections are easily recognized. The use of cytochemical and immunochemical methods and functional studies, in addition to simple morphology, may be useful in subclassification of lymphoreticular neoplasms.