The pro region of carboxypeptidase Y (CPY) from yeast is necessary for the correct folding of the enzyme [Winther, J. R., & Sørensen P. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 9330-9334]. Using fluorescence, circular dichroism, and heteronuclear NMR analyses, it is demonstrated that the isolated pro region is a partially folded protein domain under the conditions where it is functional. It is characterized by a relatively high content of secondary structural elements but a very low content of defined tertiary structure. Although these characteristics are reminiscent of the compact denatured states that have been identified as intermediates in protein folding ("molten globules"), the pro region exhibits only very weak binding of the hydrophobic probe 1-anilino-8-naphthalenesulfonate, and it is resistant toward complete thermal unfolding. Altogether the data indicate an extremely flexible structure that has little regular structural core. It is proposed that the feature of a partially folded domain per se is important for the function of the pro region of CPY as a "co-translational chaperone".