Lectin distinction of benign from malignant histiocytes. 
Ricinus communis agglutinin (RCA) staining of malignant histiocytosis (two cases) and tissue infiltrates of monoblastic leukemia revealed two distinct macrophage-histiocyte types: one with cytoplasmic staining (RCA+), the other without (RCA-).  RCA+ cells corresponded to benign-appearing histiocytes without nuclear atypia, whereas RCA-cells were cytologically malignant.  In one case of malignant histiocytosis, many RCA-tumor cells were surrounded by a thin cytoplasmic extension of RCA+ cells, which suggested an interaction between the tumor cells and stromal macrophage-histiocytes.  These observations support the view that most benign-appearing histiocytes in malignant histiocytosis are reactive in nature and suggest that RCA staining can be useful in distinguishing between benign and malignant disorders of macrophage-histiocytes.