|(translated):||The cytokines of the IL-6-type family, IL-6, IL-11 and CNTF utilize during signal transduction specific alpha-receptors, which consist of one N-terminal immunoglobulin-like (Ig) domain and two C-terminal fibronectin-type-III-like domains in the extracellular part. On target cells, interleukin-6 first binds to its specific alpha-receptor and subsequently to a homodimer of the signal transducer protein gp130. Whereas the N-terminal Ig-domain is not involved in ligand binding, the third membrane proximal domain (IL-6R-D3) accounts for more than 90% of the binding energy to IL-6. In the presented work samples of 15N- and 15N/13C-labeled IL-6R-D3 were expressed in E. coli, refolded and purified. Concentrated protein samples were prepared which are suitable for heteronuclear, multidimensional NMR spectroscopy. The analysis of 1H/15N/13C-NMR spectra provided the basis for secondary structure analysis as well as for direct determination of the IL-6 binding epitope of the IL-6R-D3. Furthermore these results are the basis for calculation of the high resolution NMR structure of IL-6R-D3.