The effects of urea on the dissociation and structural transitions of clathrin (8 S) have been evaluated by various techniques. The dissociation of the light chains in 3 M urea has been shown by light scattering, ultracentrifugation, and column chromatography. The dissociated components still retain the capacity to form the characteristic polygonal structure of the coat after removal of the urea. At higher concentrations of urea, the secondary and tertiary structures are eliminated, as documented by various spectroscopic techniques, i.e., tryptophan polarization and emission maxima, circular dichroism, and difference spectra. Two distinct transitions are observed by all techniques, one between 3 and 6 M urea and a second one which starts at 7 M but is still incomplete by 9.6 M urea. A concentration-dependent aggregation of clathrin chains occurs in 4 and 5 M urea solutions, as observed by light scattering and sedimentation. The results indicate that there are two large, independent domains in clathrin heavy chains and that each domain may have a single, highly cooperative transition.