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Cross-polarization magic-angle-spinning nuclear magnetic resonance spectroscopy has been used to determine insect cuticle composition and cross-link structure during sclerotization or tanning. Unsclerotized cuticle from newly ecdysed pupae of the tobacco hornworm, Manduca sexta L., had a high protein content with lesser amounts of lipid and chitin. Concentrations of chitin, protein, and catechol increased substantially as dehydration and sclerotization progressed. Analysis of intact cuticle specifically labeled with carbon-13 and nitrogen-15 revealed direct covalent linkages between ring nitrogens of protein histidyl residues and ring carbons derived from the catecholamine dopamine. This carbon-nitrogen adduct was present in chitin isolated from cuticle by alkaline extraction and is probably bound covalently to chitin. These data support the hypothesis that the stiffening of insect cuticle during sclerotization results primarily from the deposition of protein and chitin polymers and their crosslinking by quinonoid derivatives of catecholamines.