The stability of the folded conformation of ribonuclease T1 is increased by 0.8, 1.8, and 3.3 kcal/mol in the presence of 0.1 M NaCl, MgCl2, and Na2HPO4, respectively. This remarkable increase in the conformational stability results primarily from the preferential binding to the native protein of one Mg2+ or two Na+ ions at cation-binding sites and by the binding of one HPO4(2-) ion at an anion-binding site. Only modest binding constants, 6.2 (Na+), 155 (Mg2+), and 282 M-1 (HPO4(2-)), are required to account for the enhanced stability. One important goal of the modification of proteins through genetic engineering is to increase their stability. Our results suggest that the creation of specific cation- and anion-binding sites on the surface of a protein through amino acid substitutions might be a generally useful way of achieving this goal. The design of these sites will be aided by the recent availability of detailed structural information on cation- and anion-binding sites.