AbstractLarge sialoglycoprotein of human lymphocytes (L‐LSGP) from thymocytes and from peripheral blood lymphocytes (PBL) of normal donor and of B chronic lymphocytic leukemia (CLL) patients was purified by affinity chromatography to Mucluru pornifera agglutinin (MPA)‐Sepharose followed by preparative sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE). L‐LSGP from the three different sources was very similar in amino acid composition. It contained a high proportion of acidic and hydroxy amino acids and also significant amounts of cystein. No reduction in mobility in SDS‐PAGE was noted for unreduced L‐LSGP. The molecule may already in its native form have an extended conformation containing either free sulfhydryl groups or small S‐S loops not affecting mobility in SDS‐PAGE. L‐LSGP was found to be highly glycosylated, the thymocyte glycoprotein containing somewhat less carbohydrate by weight (44%) than that of PBL (normal PBL 53% and B CLL 52%). This was due primarily to a lower content of sialic acid. The molecules contained mannose, galactose, N‐acetyl galactosamine, N‐acetylglucosamine and sialic acid in molar ratios 1.0:3.0:1.1:1.2:1.3 (thymocyte L‐LSGP), 1.0:3.8:1.2:1.0:1.7 (PBL L‐LSGP) and 1.0:3.5:2.2:1.3:2.8 (B CLL L‐LSGP). The weak interaction of L‐LSGP with lentil lectin, concanavalin A (Con A) and leucoagglutinin (La), its unchanged mobility in SDS‐PAGE after tunicamycin treatment and its high amount of hydroxy amino acids suggest that most carbohydrate chains are O‐glycosidically linked to the peptide chain. Native as well as Nase‐treated L‐LSGP show size microheterogeneity. This is probably due to small chemical differences in the L‐LSGP molecules from different lymphocyte subsets.