-A confocal fluorescence microscope using fluo-3 and 9-(dicyanovinyl)-julolidine (DCVJ) was used to study the mast cell activation by the N-acetyl glucosamine oligomer specific lectin Datura stramonium agglutinin (DSA) and inhibition by antagonist lectins having affinity to N-acetyl glucosamine G1cNAc oligomer specific lectin DSA is a useful tool for examining the pertussis toxin-sensitive mechanisms of histamine release from rat peritoneal mast cells, as previ ously described (1). The G1cNAc-specific lectins WGA and STA were antagonists of DSA and compound 48/80. They may occupy sugar residues of glycoproteins that serve as binding sites for DSA, which overlap one of the binding sites of compound 48/80, prevent access of DSA and compound 48/80 to the binding sites and inhibit the histamine release induced by DSA and compound 48/80 (1-3). Con A, however, did not interfere with the inter actions and did not inhibit the histamine release, since it recognizes high-mannose sugar-residues but not G1cNAc oligomer (1, 3).DSA releases histamine (1), but it is not clear whether the increase in [Ca"]; is one of the initial signals trigger ing histamine release or whether the histamine releasing mechanisms are coupled with changing cytoskeletal assem bly. Confocal fluorescence microscopy, a new technol ogy, revealed important information about cell activation. Real time changes in the fluorescence of indicator dyes and fluorescent molecular rotors in living cells can be measured. In this study, we used the calcium ion indicator fluo-3 and the fluorescent molecular rotor DCVJ. Fluo-3 is a useful dye, but intracellular calcium ion concentra tion can not be calculated from its fluorescence. The inten sity of DCVJ fluorescence increased with the increasing cytoskeletal assembly of actin and tubulin molecules, and cytochalasin D inhibited the increase in the DCVJ fluores cence intensity in activated mast cells (4). We also exam ined the mechanisms of action of antagonist lectins (1) using real time measurement. To clarify the mechanisms of the DSA-induced histamine release, confocal fluores cence microscopic imaging using Con A and the antago nist lectins WGA and STA was carried out.