We have tested the hypothesis that some phenotypic characteristics of the lymphocytes from mice with lymphoproliferative disease (Ipr) could be explained by abnormal glycosylation of membrane proteins. Lymph node cells from normal C57 BL/6 and from C57 BL/lpr mice were labelled with tritiated sugars. Membrane proteins were released with trypsin. then with pronase. After complete pronase digestion, glycopeptides were first separated on Bio Gel P-6 and then on Con A-Sepharose. Fractions not binding to Con A (Con A negative) were also separated onLens culinarisagglutinin-Sepharose. Marked differences between normal and Ipr cells were noticed. First, there were more glucosamine-labelled peptides with very high molecular weight (eluting fast on Bio Gel P-6) on Ipr cells than on normal lymphocytes. Second, the proportion of mannose-labelled peptides binding to Con A was smaller in the Ipr cells. Third, among the Con A negative peptides. the proponion binding toLens culinarisagglutinin was higher in Ipr cells. Thus, Ipr cells seem to carry moreα1–6 fucosylated chains and larger size carbohydrates. These alterations were also confirmed by gel electrophoresis of lectin-selected iodinated cell surface antigens and seem to be restricted to a very limited number of peptides. Thus, there may be primary changes in glycosylation in Ipr cells. Alternatively, the glycosylation pattern of Ipr cells may be characteristic for a subpopulation of T-lymphocytes that is expanded in this disease, or for a certain stage of activation. A large proportion of Con A-negative,Lens culinaris-positive peptides is a rather unusual feature in murine cells and requires further investigation.