The one-electron reduction of hen egg white lysozyme has been reinvestigated by gamma-radiolysis using CO2.- free radicals as reductants. We show that the reaction is specific toward one out of the four disulfide bridges, i.e. the 6-127 one. This bond is in interaction with the charged end of arginine 5. The reduction leads to thiol functions and to a lesser extent to fragmentation of the polypeptide chain, which can only come from electron migration from disulfide. To get a better insight into the mechanism induced by electron transfer to the protein, the 6-127 disulfide bridge and the charged end of arginine 5 in lysozyme were modelized by R2S2 (R = H, CH3) and C(NH2)(3)(+), and ab-initio calculations were performed. All separate molecular and radical entities resulting from the electron addition were optimized with two basis sets (6-31G* and 6-31+G*) and at the MP2 correlation level. The formation of complexes was studied and four zwitterionic and two neutral radical complexes involved in charge transfer reaction were characterized at the MP2 level. The influence of the environment was taken into account by using the Onsager reaction field method (SCRF) for the isolated species as well as the complexes.