A sialic acid-binding lectin (Hericium erinaceum lectin, HEL), isolated from fresh fruiting bodies of Hericium erinaceum, was treated with various temperature and pH to investigate its fluorescence spectra and hemagglutinating activity. It was found that the hemagglutinating activity of HEL was relatively steady below 60 degrees and at pH from 6 to 11, and the change of hemagglutinating activity was relative to the change of hydrophobic areas where tryptophan residues located. In fluorescence quenching study of HEL by acrylamide and KI, it was indicated that nearly all the tryptophan residues of HEL located on the surface of the molecule, and most of them were in hydrophobic areas or negatively charged areas. Chemical modification of HEL proved that there were about twelve tryptophan residues in a HEL molecule and all of them were located on the surface or in the shallow groove of the molecule, and eight of them were essential for hemagglutinating activity; aspartic acid or glutamic acid residues were involved in maintaining the crucial conformation of activity center and made great contribution to the hemagglutinating activity of HEL, but they could not touch the sialic acid molecule directly; tyrosine residues also played a role in the hemagglutinating activity of HEL; while arginine, serine, threonine, histidine residues had no effect on the hemagglutinating activity of HEL.