The urea denaturation of Bence Jones proteins, one type κ(Ta) and two type λ (Fu and Ni), has been studied by measurements of optical rotation, circular dichroism (CD), ultraviolet absorption, and the reactivities of tryptophyl residues and disulfide bonds. In the case of Ta protein, the changes with urea concentration in the optical rotation at 250 nm, the CD spectrum and the reactivity of the intrachain disulfide bonds toward dithiothreitol occurred in at least two stages and no complete transition was observed even with 8 M urea. On the other hand, both the tryptophyl residues had reacted with 2-hydroxy-5-nitrobenzyl bromide in 4 M urea. The transition curve, measured by ultraviolet absorption spectroscopy, was characterized as sigmoidal. On the basis of these findings and the observation by Titani et al. (J. Biol. Chem., 244, 3521 (1969)) that the disulfide bond in the variable half of a κ protein resists reduction, it was suggested that the unfolding of the constant half and that of the variable half occur simultaneously and the intermediate observed in the course of urea denaturation can be characterized as the molecule in which the constant half is completely unfolded and a portion of the variable half which contains a disulfide bond remains folded. The change in the CD spectrum of Ni protein with urea concentration showed that the constant half is less stable than the variable half, but a conformational change in the variable half begins to occur prior to completion of the unfolding of the constant half. In the case of another type λ protein, Fu, there was no distinct difference between the stabilities of the constant and variable halves. For both λ proteins, the reactivity of the intrachain disulfide bonds increased before the changes in the optical rotation and absorption were observed.