The solubility of a protein can be defined as the concentration of the soluble protein in equilibrium with the solid phase under given conditions of pH, temperature, buffer, and/or various additives. A solubility curve represents the dependence of this saturating concentration on a single parameter, which could be the concentration of a crystallizing or precipitating agent. For accurate determinations, the solubility should be determined by both crystallization and redissolution of crystals, and it should be demonstrated that in the two methods the protein concentration converges asymptotically to the same value. In cases where fairly large amounts of protein are readily available, solubility diagram analysis provides a means of quantitatively characterizing the effect of one parameter, and hence of interpreting its influence in a meaningful way. Our analysis of lysozyme (pI = 11.1) solubility curves in presence of various salts, at pH 4.5 and 18°C, indicates that solubility is affected more by anions than by cations and that the effectiveness of the anions is in an order roughly the reverse of that of the Hofmeister series. Under these conditions, thiocyanate, a chaotropic anion at high concentration, is found to be a very effective crystallizing agent at low (<0.1 m) concentrations. Two other monomeric proteins (erabutoxin and bovine pancreatic trypsin inhibitor) of high isoelectric point crystallized also at pH 4.5 and 18°C, confirming the much higher effectiveness of KSCN compared to NaCl. The width of the crystallization zone between the solubility curve and the curve above which only precipitation occurs also varies considerably with different anions. These observations may have useful implications in screening and optimizing crystallization conditions.