The constant fragment of the immunoglobulin light chain in which the intramolecular disulfide bond is reduced (reduced CL fragment) assumes a conformation very similar to that of the intact CL fragment and contains two sulfhydryl groups buried in the interior of the molecule [Goto, Y., & Hamaguchi, K. (1979) J. Biochem. (Tokyo) 86, 1433-1441]. In order to understand the role of the disulfide bond, a derivative in which the disulfide bond is replaced by an S-Hg-S bond was prepared and its conformation and stability were studied. The derivative was prepared by reacting the reduced CL fragment with mercuric chloride. Kinetic studies showed that the reaction is rate-limited by the unfolding process of the reduced CL fragment. The mercury derivative was as compact as the intact CL or reduced CL fragment, and a tryptophyl residue was found to be buried near the S-Hg-S bond in the interior of the protein molecule. Judging from the circular dichroic spectrum, however, the beta-structure characteristic of the immunoglobulin fold was disturbed. The stability of the derivative to guanidine hydrochloride was lower than that of the intact CL fragment, but the unfolding transition was reversible and cooperative. Decreased stability of the mercury derivative is due to its folded conformation being distorted by introduction of the S-Hg-S bond.