BACKGROUND It has long been established that temperature-induced melting of small globular proteins is an all-or-none transition. Little was known, however, about the degree of cooperativity of denaturant-induced transitions in proteins, especially in those cases in which the proteins unfold through the molten globule state. RESULTS We have processed data on the equilibrium urea-induced and guanidinium chloride (GdmCl)-induced unfolding of globular proteins from the native to the unfolded state, from the native to the molten globule state and from the molten globule to the unfolded state. We show that in all these cases, the cooperativity of unfolding increases linearly with the increase of the molecular weight of the protein up to 25-30 kDa. CONCLUSIONS The cooperative unit of the urea-induced and GdmCl-induced equilibrium transitions of small proteins between the native, molten globule and unfolded states includes the protein molecule as a whole. In other words, both native and molten globule proteins are unfolded by strong denaturing solvents according to an all-or-none mechanism.