Denaturation of uridine phosphorylase from Escherichia coli K-12 by guanidine hydrochloride results in red shift of the maximum in the protein fluorescence spectrum, dissociation of the hexameric enzyme molecule into monomers, and the loss of the enzymatic activity. The initial rate of the enzyme reactivation after the dilution of the enzyme preincubated with guanidine hydrochloride has the second order with respect to protein. It is assumed that the rate of the reactivation process is limited by the reassociation of monomers possessing low enzymatic activity to dimers followed by the rapid step of hexamer formation.