A process for the production of a solubilized protein is disclosed, which is obtained by reducing disulfide bonds in a disulfide bond-containing water-insoluble protein material into mercapto groups and subsequently converting a part or an entire portion thereof into carboxymethyldisulfide groups. In particular, the disclosed process for the production of a solubilized protein which comprises (a) treating a disulfide bond-containing water-insoluble protein material with an aqueous alkaline solution of a reducing agent, and (b) reacting the protein treated by step (a) with thioglycolic acid in the presence of an oxidizing agent under a weakly acidic to a weakly alkaline condition. A process for the production of regenerated protein products, which comprises regenerating disulfide bonds in the solubilized protein, is also disclosed.