We propose to use rotational-echo, double- and triple-resonance solids NMR to measure interatomic distances between stable-isotope labels in amino-acid residues at or near binding sites of large proteins, and stable-isotope labels in substrate analogues or inhibitors bound to the proteins. These experiments can be performed on one multi-labeled site per 600 residues and will measure C-N, C-P, and C-F distances as great as 5, 8, and 12 A, respectively, with an accuracy of +/-0.5 A. Collaborations with five laboratories have been established to perform labeling of the proteins. The proteins to be examined include (1) rat intestinal fatty-acid binding protein, (2) methanol dehydrogenase, (3) 5-enolpyruvylshikimate-3-phosphate synthase, (4) plant glutamine synthetase, (5) thermolysin, and (6) tropomyosin. Analysis of these proteins and their complexes in both lyophilized and microcrystalline forms will be aided by continued development of new analytical techniques including (1) weak-pulse rotational-echo double resonance (REDOR), (2) many-cycle REDOR, (3) 17 O 13C REDOR, and (4) homonuclear REDOR. Computer simulations of rotational-echo triple-resonance (RETRO) NMR experiments will yield information about distances and orientations of clusters of three stable-isotope labels. RETRO NMR permits a general approach to the problem of identifying single sites in large, immobilized protein complexes with total [suppression of interferences from the natural-abundance background.