Rotational-echo double-resonance (REDOR) 13C NMR spectra with 2H dephasing have been obtained from plaques and threads from the byssus of the marine mussel Mytilus edulis labeled by sea-water exposure to L-[ring-4-(13)C]tyrosine and L-[ring-d4]tyrosine for 2 days. Specific isotopic enrichment of tyrosine in protein reached 25% in both 13C and 2H. Fifteen percent of the total 13C label was incorporated as diphenolic carbon. Based on REDOR dephasing, about one-tenth of tyrosine rings in both intact plaques and threads are within 4 A of each other or rings of 3,4-dihydroxy-phenylalanine (DOPA). However, there is no direct evidence for the formation of covalent linkages between or among tyrosine and DOPA rings in either plaques or threads.