Pig kidney fructose-1,6-bisphosphatase is a homotetrameric enzyme which does not contain tryptophan. In a previous report the guanidine hydrochlo-ride-induced unfolding of the enzyme has been described as a multistate process [Reyes, A. M., Ludwig, H. C., Yan˜ez, A. J., Rodriguez, P. H and Slebe, J. C.