Differentiation of most T lymphocytes is characterized not only by the variable expression of CD4/CD8 coreceptor molecules and increased surface density of the T cell antigen receptor, but also by changes in the glycosylation pattern of cell surface glycolipids or glycoproteins. In this work we evaluated the changes in the sialylation pattern in thymus sections from normal and dexamethasone treated mice. We used sialic acid specific lectins, such as Sambucus nigra agglutinin (SNA, NeuAcalpha2,6-Gal specific) and Maackia amurensis agglutinin (MAA, NeuAcalpha2,3-Gal specific). Our results indicate that the sialylation pattern was modified during the maturation process of thymic cells. The immature CD4-CD8- and CD4+CD8+ cortical thymocytes were recognized by SNA, whereas the mature single positive (CD4+ or CD8+) medullary cells, preferentially bound MAA lectin. However, in the corticomedullary region we found not only SNA+ cells, but also MAA+ cells. In the thymus of dexamethasone treated mice, the clusters of thymocytes undergoing apoptosis in the cortex were characteristically stained by SNA. These results suggest that in the initial stages of the differentiation pathway, a great number of thymocytes express an alpha2,6 linked sialic acid on their surface and as they progress to more mature stages there is a change in the sialylation pattern to alpha2,3 linked sialic acids probably due to a regulated expression of different sialyltransferases, which could be modulated by the thymic microenvironment.