The constant (CL) domain of the type-lambda immunoglobulin light chain contains two tryptophyl residues. The fluorescence lifetimes of the CL fragment and its two modified forms, the reduced CL, and reduced and alkylated CL fragments, were measured at pH 7.5 and 25 degrees C. The fluorescence decay kinetics of these fragments were described in terms of two lifetimes. The static and dynamic quenching reactions by acrylamide of the tryptophyl residues of these fragments were measured and their dynamic properties were compared with the static properties reported previously (Goto, Y. & Hamaguchi, K. (1979) J. Biochem. 86, 1433-1441; Goto, Y. & Hamaguchi, K. (1986) Biochemistry 25, 2821-2828; Ashikari, Y., Arata, Y., & Hamaguchi, K. (1985) J. Biochem. 97, 517-528). It was found that although the static conformation of the reduced CL fragment is very similar to that of the intact CL fragment, the dynamic conformations of these two fragments differ considerably.