The C-13-enriched human hair was prepared by labeling human hair with C-13-enriched methyl iodide for the thiol group formed by reduction of the disulfide bonds. In the C-13 CP/MAS n.m.r. spectrum, the Cys S-(CH3)-C-13 peak split into two peaks with a chemical shift difference of 1.7 ppm. With increasing reduction time in the process of the sample preparation, the relative intensity of the down held peak is increased from 0.17 (20 min reduction time) to 0.53 (20 h reduction time), although that of the up field peak stays almost constant. Here the main peak of the carbonyl carbon was used for the reference of the peak intensity ( = 1.00). On the other hand, the residual ratio of the disulfide bond is decreased from 15% at the reduction time of 20 min to approximately 0% at that of 20 h, These results indicate that the down field peak is assigned to the Cys S-(CH3)-C-13 carbons in the domain with the ordered structure where the disulfide bond is relatively difficult to reduce during the sample preparation. Besides, the up field peak is assigned to the Cys S-(CH3)-C-13 carbons in the amorphous domain where the reduction occurs completely for at least 20 min. These two Cys S-(CH3)-C-13 peaks are remarkably different in the half-height width. From the determination of the C-13 spin-lattice relaxation times of two peaks, this difference is due to a difference in the chemical shift distribution rather than the difference in the molecular motion.