The denaturation and renaturation by guanidine hydrochloride of Fc(t) fragment whose interchain disulfide bonds are reduced and alkylated (R.A.Fc(t)) and pFc' fragment of a human myeloma protein (IgGl, kappa) were studied using tryptophyl fluorescence. R.A.Fc(t) was found to consist of a slow-unfolding region and a rapid-unfolding region. The denaturation of pFc' was extremely slow. Comparison of the kinetic and equilibrium data of the denaturation of R.A.Fc(t) with those of pFcl indicated that the slow-unfolding region of R.A.Fc(t) corresponds to the CH3 region and the fast-unfolding region the CH2 domain. This was also confirmed by the analysis of the CD and fluorescence spectra for R.A.Fc(t) and pFc' at various concentrations of guanidine hydrochloride. Although the kinetic stability of the CH3 region was much higher than that of the CH2 region, the thermodynamic stabilities of these domains were almost the same; the free energy change of the denaturation in water being about 6 kcal . mol-1. This value is also the same as the value for the CL fragment (Goto, Y. & Hamaguchi, K. (1979) J. Biochem. 86, 1433--1441). It was suggested that the high kinetic stability of the CH3 region in R.A.Fc(t) is due to the strong tendency for the CH3 domains to form a dimer.