The known cross-links of bone collagen are derived from lysine and hydroxylysine. The first step in the enzymatic cross-linking process is a deamination by lysyl oxidase producing an aldehyde which then may condense with a lysyl or hydroxylysyl residue of a neighbouring collagen molecule. Some of the resulting divalent aldimine and oxo-imine cross-links may later on be incorporated in trivalent hydroxylysyl-pyridinoline and lysyl-pyridinoline cross-links. In bone collagen prepared from the cancellous bone of vertebral bodies of osteoporotic individuals we found a reduced stability towards acetic acid and pepsin, and a substantial reduction in the concentration of the divalent collagen cross-links compared with sex- and age-matched controls. To what extent do the collagen cross-links influence the mechanical properties of bone? β-amino-propionitrile (BAPN) irreversibly inhibits the enzyme lysyl oxidase and therefore, the formation of cross-links between the collagen molecules. In the present study female rats, 70 days old, injected subcutaneously two times daily with BAPN (333 mg/kg/day) for 1 month and saline injected control rats were studied. The concentration of the hydroxypyridinium cross-links of femoral mid-diaphyseal cortical bone was determined by HPLC with fluorescence detection and the mechanical properties of the rat femoral diaphyses were analyzed by a materials testing machine. The BAPN injections resulted in a 45% reduction in the concentration of the hydroxypyridinium crosslinks and a 31% decrease in the stability of the bone collagen towards acetic acid and pepsin compared with the control rats. No changes were found in ash or collagen concentrations of the cortical bone. The mid-diaphyseal deflection until breaking, bending stress and elastic stiffness (Young's modulus) of the BAPN injected rats were decreased by 21%, 26% and 30%, respectively, compared with the control rats. The present study shows that collagen cross-links are essential for bone to possess a sufficient deflection capacity, bending strength and stiffness.