In mice a single injection of 4 mg of Dextran sulphate within a few days produces an enlargement of area draining lymph nodes and activates basophils/mast cells. This activation is manifested by many-fold increase of mast cell number per lymph node, degranulation of mast cells and appearance of young, immature mast cells capable to synthetize new granules. In contrast to the lymph nodes, the Dextran-injected connective tissue mast cells remained unchanged. This suggest that Dextran sulphate directly activates lymph node mast cells, probably by activation of T-cell suppressor or macrophages.