Human uteroglobin (h-UG) or Clara cell 10kDa (cc10kDa) is a steroid-dependent, 17 kDahomodimeric, secretory protein with potent anti-inflammatory/immunomodulatory properties.However, the exact physiological role still remains to be determined. It has been hypothesisedthat its activity is exerted through the binding of a specific target represented by a smallmolecule (still unknown), and that the binding is regulated by the formation/disruption of twocysteine bonds. The binding properties of the reduced UG have been proved in vitro forseveral different molecules, but no in vivo data are available to date. However, binding hasbeen observed between reduced rabbit UG and a protein of an apparent molecular mass of90 kDa and, more recently, we found an h-UG-binding protein (putative receptor), of anapparent molecular mass of 190 kDa, on the surface of several cell types. The recognitioninvolves oxidised h-UG. These findings pose the problem of the relevance of the oxidationstate in the recognition process. To determine the solution structure of the oxidised h-UG, weproduced wild-type as well as uniformly 15N- and 15N/13C-labelled samples of therecombinant protein. The assignments of the 1H, 15N and 13C resonances are presented,based on a series of homonuclear 2D and 3D and heteronuclear 2D and 3D double and tripleresonance NMR experiments. Our results indicate that h-UG is an extremely stable proteinunder a wide range of temperatures and pH conditions. The secondary structure in solutionis in general agreement with previously reported crystal structures of rabbit UG, suggestingthat cc10kDa and h-UG are indeed the same protein. Small local differences found in the N-and C-terminal helices seem to support the hypothesis that flexibility involves these residues;moreover, it possibly accounts for the residual binding properties observed when the proteinis in the oxidised state.