A lactose-binding lectin (Agrocybe cylindracea Lectin, ACL) purified from fruiting bodies of the mushroom A. cylindracea was investigated to determine the hemagglutinating activity and conformation changes after chemical modification, removal of metal ion and treatment at different temperatures and pH. ACL agglutinated both rabbit and human erythrocytes and its hemagglutinating activity could be inhibited by lactose. This lectin was stable in the pH range of 6-9 and temperature up to 60 degrees C. Fluorescence quenching and modification of tryptophan residues indicated that there were about two tryptophan residues in ACL molecule and one of them might be located on the surface, while the other was buried in the hydrophobic shallow groove near the surface. Chemical modification of serine/threonine and histidine showed that the partial necessity of these residues for the hemagglutinating activity of ACL. However, modifications of arginine, tyrosine and cysteine residues had no effect on its agglutinating activity.