The proteins in the distensible alloscutal cuticle of the blood-feeding tick, Ixodes ricinus, have been characterized by electrophoresis and chromatography, two of the proteins were purified and their total amino acid sequence determined. They show sequence similarity to cuticular proteins from the spider, Araneus diadematus, and the horseshoe crab, Limulus polyphemus, and to a lesser extent to insect cuticular proteins. They contain a conserved sequence region, which is closely related to the chitin-binding Rebers-Riddiford consensus sequence present in many insect cuticular proteins. Only a fraction of the alloscutal proteins can be readily dissolved, and the dissolved proteins are difficult to separate by electrophoresis and column chromatography. The insoluble fraction can only be dissolved after degradation to smaller peptides. The mixture of extractable proteins as well as hydrolysates of the insoluble fraction are fluorescent when exposed to ultraviolet light, and the fluorescence corresponds in excitation and emission maxima to the fluorescence of the rubber-like arthropodan protein, resilin, and to the amino acid dityrosine. Small amounts of dityrosine were obtained from ticks in the early phase of a blood meal when the cuticle weighs less than 4 mg; increasing amounts were obtained from animals in the initial period of feeding, during which the cuticular weight increases from 4 to 11 mg, whereas little increase in dityrosine content was observed during the final period of engorgement. Cuticle from fully distended ticks contains about 60-80 nmole dityrosine per tick, corresponding to 2-3 microg/mg cuticle. It is suggested that the major part of the cuticular proteins is made inextractable by cross-linking by dityrosine residues, and that dityrosine plays a role in stabilizing the cuticular structure during the extensive distension occurring during a blood meal. Small amounts of 3-monochlorotyrosine and 3,5-dichlorotyrosine were obtained from the distended tick cuticle, corresponding to chlorination of between 0.5% and 1.5% of the tyrosine residues. It is suggested that the chlorotyrosines are a side-product of oxidative processes in the cuticle.