AbstractChickens were immunized with highly purified large sialoglycoprotein of human lymphocytes (L‐LSGP; gp 150) which was isolated from neuraminidase‐treated normal peripheral blood lymphocytes by affinity chromatography to HP‐Sepharose and further purified by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. Antibodies isolated from plasma and egg yolk were highly specific for desialylated L‐ LSGP (apparent molecular mass ∼ 150 kDa). The antigenic sites recognized by the antibodies are probably located in the peptide moiety of the molecule since antibody binding to lymphocytes was not inhibited by a variety of different sugars or abrogated by absorption on various erythrocytes.In immunofluorescence experiments, ⩾ 75% of neuraminidase‐treated thymocytes and peripheral blood lymphocytes, virtually all E+ cells and T4+ or T8+ T chronic lymphocytic leukemia (CLL) cells were stained by anti‐gp150. A small fraction (∼ 10%) of thymocytes and a larger fraction (⩾ 30%) of T CLL cells in some patients were stained before neuraminidase treatment. Thymocytes appear to contain considerably lower amounts of a less sialylated form of L‐LSGP than peripheral blood lymphocytes. In contrast to blast cells of 5‐day concanavalin A or leucoagglutinin cultures (⩾ 90% anti‐gpl50+) only about 50% of the blast cells generated in 5‐day mixed leukocyte cultures were anti‐gpl50+. The large majority (⩾ 75%) of both the anti‐gpl50+ and anti‐gpl50− cells were T3+ and T11+.About 25% of neuraminidase‐treated B cells in normal adult blood and in umbilical cord blood were stained in immunofluorescence using chicken anti‐gpl50. Of the cell lines, 0/2 with progenitor B cell phenotype (Epstein‐Barr virus‐transformed fetal liver cells), 3/5 populations exhibiting a pre‐B cell phenotype, 2/2 of somewhat later maturation and 1/3 myeloma lines were anti‐gpl50+ when stained after neuraminidase treatment. Biopsies from patients with lymphoproliferative disorders similarly showed that 7/7 CLL populations and 2/2 bone marrow biopsies obtained from multiple myeloma patients were anti‐gpl50+, whereas prolymphocytic leukemia cells were anti‐gpl50−. We propose therefore that gp150 may be associated with a separate lineage of B cells rather than expressed during the early stages of maturation.L‐LSGP was not found on peripheral blood monocytes, granulocytes, platelets or erythrocytes. Rabbit, rat and mouse lymphocytes appear to contain a glycoprotein very similar to human L‐LSGP. It is probably a conservative molecule showing very little species specificity in mammals.It is suggested that one important role of L‐LSGP may be to maintain cell integrity during the circulatory life of the lymphocyte.