To understand more precisely the function of free glutenin SH and SS groups in glutenins of developing wheat for UPP formation, a specific sulfhydryl probe, monobromobimane (mBBr), was used for an in vitro protein labeling. By applying this procedure to two varieties of wheat differing in high molecular weight glutenin subunit composition (2*, 7 + 8, 5 + 10 and 0, 6 + 8, 2 + 12, respectively, for Soissons and Thésée), we showed that the major wheat glutenin subunits residing in the protein body undergo redox change during the development and the maturation of the grain. Indeed, during the cell division and the cell enlargement phase, glutenin subunits and particularly LMW-GS have a large amount of free SH groups and become oxidized during grain dehydration which coincided with the formation of UPP. Moreover, mBBr derivatization of free glutenin SH groups before the artificial grain desiccation inhibits the UPP deposition. As HPSEC-MALLS analysis showed, the alkylation of free glutenin SH groups before the desiccation induces an increase in the SDS solubility of the polymeric proteins by reducing both their molecular weight distribution and their compactness. These results are discussed in connection with an 'hyperaggregation model' which has been proposed to explain the formation of glutenin polymer in wheat kernel.