A method is provided for improving the solubility of proteins, for example, bacterial toxins. In one embodiment, solubility is improved by introducing point mutations that replace cysteine residues capable of forming intermolecular disulfide bonds with other amino acid residues that do not form such bonds. By abrogating the ability of the cysteine residues to form inter-molecular disulfide bonds, aggregation of the protein is reduced, thereby improving the solubility of the protein. In another embodiment, solubility of the protein is improved by producing truncated forms of the protein that express the LHN domain and a fragment of the Hc domain. Proteins made according to the method of the invention are useful, for example, as immunodiagnostic agents and vaccine components.