The folding pathway of a protein molecule in vivo is related to the formation of disulfide bonds. However, little is known about the mechanism of the folding process, because it is quite difficult to identify an intermediate in the formation of disulfide bonds during protein folding in vivo. Here, the relationship between protein folding and disulfide bond formation is discussed based on the results of the structural analyses of mutant human lysozymes, designated as C77/95A and C77A-a, in which a disulfide bond between Cys77 and Cys95 was modified.