The Lewis X antigen. A new paraffin section marker for Reed-Sternberg cells. Using a monoclonal antibody specific to the Lewis X antigen (anti-Lex), the authors studied 103 cases of Hodgkin's disease (HD) in comparison with 57 cases of non-Hodgkin's lymphoma (NHL); three cases of granulocytic sarcoma (GS); two cases of malignant histiocytosis (MH); one case of monoblastic leukemia (ML); one case of interdigitating reticulum cell sarcoma (IRCS); six cases of histiocytosis X (HX); one case of reticulohistiocytoma (RH); 44 various reactive conditions of the lymph node (LN). Reed-Sternberg and related (R-S) cells stained selectively in 80 of 92 cases of HD (87.0%), excluding 11 cases of lymphocyte predominance type. The stain was better in B-5-fixed specimens than in formalin-fixed specimens, showing a dense deposit of reaction products at a paranuclear site and on the cell surface. The staining results were compared with those of Leu-M1 and found to be superior both qualitatively and quantitatively (detection rate of R-S cells: 87.0% versus 68.5% of Leu-M1). Granulocytes, rare epithelioid histiocytes, and some endothelial and/or erythrocytes also stained with anti-Lex. The stain had positive results in three cases of GS showing a diffuse cytoplasmic staining pattern. Of NHL, two of 29 peripheral T-cell lymphomas stained to show rare paranuclear deposits without cell surface staining. The stain had negative results in MH, ML, IRCS, HX, and RH. Of 45 reactive LN, minute subcapsular collections of Lewis X+, altered-appearing Langerhans'-like cells, were observed in all ten LN from human immunodeficiency virus (HIV)-associated persistent generalized lymphadenopathy (PGL). The stain had negative results in all other various reactive conditions of LN. In conclusion, Lewis X staining is useful as a marker for R-S cells in paraffin sections with staining results superior to those of Leu-M1. Lewis X staining also detects subcapsular clustering of altered-appearing Langerhans'-like cells in PGL, which has not been described previously and warrants additional study. 