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Proton-decoupled Fourier transform nuclear magnetic resonance of natural abundance 13C was used to obtain spectra of cyanoferrimyoglobins of harbor seal (Phoca vitulina) and sperm whale (Physeter catodon). These spectra were compared with those of hen egg white lysozyme and of bovine pancreatic ribonuclease A to show differences reflecting gross composition as well as differences tentatively attributable to more subtle conformational constraints. The cyanoferrimyoglobin spectra were much alike but not identical. Carboxymethylation of the native seal protein produced barely detectable changes at the chemical shift positions of the major adducts but no systematic alterations elsewhere in the spectrum. However, extensive modification of histidine and methionine residues by carboxymethylation of seal apomyoglobin in the presence of 8 murea yielded a denatured product, judged by circular dichroism measurements, for which the 13C spectrum was sharp and characteristic of a denatured protein. This spectrum was used to tabulate tentative assignments. Partially relaxed spectra were used to obtain spin-lattice relaxation times (T1) for certain carbon types. These values are compared for the various myoglobin preparations mentioned above. The similarities between the native forms are striking. The denatured preparation gives many evidences of increased segmental mobility, especially for those nuclei that appear relatively restrained in the native state.