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