The role of carbohydrate structures in the interaction of lymphocytes and endothelial cells is well established. Here the influence of sialic acid in the entrance and localization of lymphocytes in the lymphoid white pulp area of the spleen was studied by injecting sialidase in vivo. A role for sialic acid molecules on stromal elements of the spleen was determined. Although the identity of the cells that bear sialidase sensitive receptors could not be established, a role for marginal zone macrophages could be ruled out by macrophage depletion studies.