A 13C NMR study is reported of the hinge region of an intact mouse monoclonal antibody with a molecular weight of 150 K. Cys, Ile, and Pro analogs of the antibody labeled with 13C at the carbonyl carbon were prepared by growing hybridoma cells in the serum-free media. Resonance assignments have been performed as described previously [Kato, K., Matsunaga, C., Igarashi, T., Kim, H., Odaka, A., Shimada, I. and Arata, Y. (1991) Biochemistry, 30, 270-278]. The spectral data obtained show that 13C NMR can give detailed information about the structure of the hinge region of the intact antibody molecule. Prospects for the future role of 13C NMR in the structural analyses of larger proteins are briefly discussed.