The second osmotic virial coefficient, B-22, of protein solutions is a function of the protein interaction potentials and, thus, summarizes many solution parameters leading to crystallization or precipitation of proteins. B-22 of hen egg-white lysozyme and bovine serum albumin was measured by static light scattering under the variation of pH, temperature, and crystallization agents. Comparing B-22 values and precipitate morphology in a systematically analyzed range of solution conditions allows the construction of the so-called crystallization slot. The thermodynamic driving force for crystallization is supersaturation, which is set by the protein concentration and the solubility (related to B-22). Hence, besides the optimal B-22 range, the protein concentration and, if solubilities are available, the supersaturation leads to a two-dimensional crystallization window where crystal formation is possible. This is shown in a generalized state diagram, where protein morphologies are solely determined as a function of protein concentration or supersaturation and interactions.