We present a new method that integrates selective biosynthetic labeling and solid-state NMR detection to identify in situ important protein cross-links in plant cell walls. We have labeled soybean cells by growth in media containing L-[ring-d(4)]tyrosine and L-[ring-4-C-13]tyrosine, compared whole-cell and cell-wall C-13 CPMAS spectra, and examined intact cell walls using C-13{H-2} rotational echo double-resonance (REDOR) solid-state NMR. The proximity of C-13 and H-2 labels shows that 25% of the tyrosines in soybean cell walls are part of isodityrosine cross-links between protein chains. We also used N-15{C-13) REDOR of intact cell walls labeled by L-[epsilon-N-15,6-C-13]lysine and depleted in natural-abundance N-15 to establish that the side chains of lysine are not significantly involved in covalent cross-links to proteins or sugars.