The assignment of the carbon-13 magnetic resonance spectrum of l-tryptophan in aqueous solution and in trifluoroacetic acid is made by comparison with the spectra of indoles, by double resonance and by deuteration experiments. On the basis of work on small peptides, it is possible to assign all the downfield resonances in denatured proteins to single types of carbon atoms, with the exception of the carbonyl carbons and the histidine Cγ resonance. In the upfield region there are only about eleven resonances which correspond to single types of carbon atoms from eight different amino acid residues. The spectra of a particular protein denatured by heat, 6 M guanidine · HCl or formic acid are all similar. The spectrum of a native protein is much broader than that of the denatured protein due to small differences in chemical shift which result from environmental changes of some carbon atoms, and line broadening due to decreased mobility of carbon atoms.