This work provides evidence of a physical instance in which some proteins that are usually inactivated under strong chaotropic conditions may become fully resistant through the occupancy of their binding sites with suitable ligands. In this regard, we found that Moluccella laevis lectin remains stable in the presence of denaturant concentrations of urea when an appropriate saccharide is bound to the protein (Alperin, D.M., Latter, H., Lis, H., and Sharon, N. (1992) Biochem. J. 285, 1-4). Extending this finding, we now demonstrate that the occupancy of the ligand binding sites of concanavalin A (Con A) with appropriate carbohydrates completely prevents the denaturation course elicited by 8 M urea at pH 7.4. In addition, the protecting efficiency of the saccharides was shown to be directly related to their specificities for the lectin. The observed saccharide protection follows the order:methyl alpha-D-mannopyranoside > methyl alpha-D-glucopyr-anoside > mannose > fructose > glucose. Concomitantly, the active tetrameric lectin with a molecular mass of approximately 105 kDa is preserved in 8 M urea when methyl alpha-D-mannopyranoside (100 mM) is present in the medium.