The structural analysis of natural protein fibers with mixed parallel and antiparallel beta-sheet structures by solid-state NMR is reported. To obtain NMR parameters that can characterize these beta-sheet structures, C-13 solid-state NMR experiments were performed on two alanine tripeptide samples: one with 100% parallel beta-sheet structure and the other with 100% antiparallel beta-sheet structure. All C-13 resonances of the tripeptides could be assigned by a comparison of the methyl C-13 resonances of Ala(3) with different [3-C-13] Ala labeling schemes and also by a series of RFDR (radio frequency driven recoupling) spectra observed by changing mixing times. Two C-13 resonances observed for each Ala residue could be assigned to two nonequivalent molecules per unit cell. Differences in the C-13 chemical shifts and C-13 spin-lattice relaxation times (T-1) were observed between the two beta-sheet structures. Especially, about 3 times longer T-1 values were obtained for parallel beta-sheet structure as compared to those of antiparallel beta-sheet structure, which could be explicable by the difference in the hydrogen-bond networks of both structures. This very large difference in T-1 becomes a good measure to differentiate between parallel or antiparallel beta-sheet structures. These differences in the NMR parameters found for the tripeptides may be applied to assign the parallel and antiparallel beta-sheet C-13 resonances in the asymmetric and broad methyl spectra of [3-C-13]-Ala silk protein fiber of a wild silkworm, Samia cynthia ricini.