The renaturation of Escherichia coli K 12 tryptophanase after treatment with 8 M urea has been studied. It is shown that, during the renaturation process, an inactive as well as an active form of the protein can be obtained. The influence on the relative amounts of these two forms of factors such as temperature, the presence of the coenzyme, the protein concentration and the conditions for removal of urea has been investigated. The results obtained indicate that the inactive form is characterized by incorrect quaternary interactions occurring specifically between tryptophanase protomers. These findings are discussed in the general frame of the mechanism of protein folding, and are taken as an evidence for the existence of nucleation centers in the folding of tryptophanase.