The conformation of the constant fragment of the immunoglobulin light chain in which the intrachain disulfide bond is replaced by the bond S-Hg-S (C_L, Hg fragment). was as compact as that of the intact C_L fragment, but its stability to guanidine hydrochloride was lower than that of the intact C_L fragment [Goto, Y. & Hamaguchi, K. (1986) Biochemistry in press]. The kinetics of reversible unfolding and refolding of the C_L-Hg fragment by guanidine hydrochloride were studied and compared with those for the intact C_L and reduced C_L fragments [Goto, Y. & Hamaguchi, K. (1982) J. Mol. Biol. 156, 891-910, 911-926]. The unfolding kinetics were explained on the basis of a three-species mechanism, U₁_??_U₂_??_F, where U₁ and U₂ are respectively slow-folding and fast-folding species of unfolded protein, and F is folded protein. However, an additional isomerization, though its contribution to the overall reaction process is small, had to be taken into account to explain the refolding kinetics. The kinetic properties of interconversion between U₁ and U₂ were similar to those for the intact C_L and reduced C_L fragments. This suggested that the same prolyl residue is involved in the isomerization reactions in the unfolded states of the intact C_L, reduced C_L, and C_L-Hg fragments. The rate constant for the unfolding process, F to U₂, was about 20 times greater than those for the intact C_L and reduced C_L fragments, while the rate constant for the refolding process, U₂ to F, lay between the values for the intact C_L and the reduced C_L fragment. The free energy profiles of unfolding and refolding of the intact C_L, reduced C_L, and C_L-Hg fragments were compared.