Structural studies of globular proteins by nmr can be enhanced by the use of isotope enrichment. We have been working with proteins enriched with 15N, and with both 15N and 13C. Due to the isotope enrichment we could assign several large proteins with up to 186 residues and could address structural questions. Furthermore, we can accurately measure heteronuclear and homonuclear vicinal coupling constants. This involves in part multidimensional multiple resonance experiments. This is important for characterization of minor conformational changes caused by mutations. We have also made use of isotope enrichment to study the internal mobility of proteins. We also have developed novel methods for measuring accurately 15N relaxation parameters, in particular transverse relaxation rates. This has led us toward a method for directly mapping spectral density functions of the rotational motions of N–H bond vectors in proteins. The protein systems that are discussed include the unlabeled proteins kistrin and cytochrome c551, and the labeled proteins eglin c, a flavodoxin, and human dihydrofolate reductase. Copyright © 1992 John Wiley & Sons, Inc.