A method for producing a native immunoglobulin composition having a high monomer content, which comprises contacting a native immunoglobulin in aqueous solution with about 10 to about 600% by weight, based on the weight of the native immunoglobulin, of a water-soluble acid addition salt of a basic amino acid having a pKb at 25.degree. C. of not more than 7 at a temperature between about 0.degree. and about 50.degree. C. in the absence of freezing and at a pH between about 5 and about 8, thereby to prevent aggregation of the monomer of the native immunoglobulin in the aqueous solution, and when the native immunoglobulin contains aggregated molecules, dissociating the aggregated molecules in the aqueous solution into monomers.