Lysyl oxidase catalyzes the formation of crosslinking aldehydes in collagen and elastin. This report demonstrates that the enzyme has high activity with collagen precipitated as native fibrils, an apparent K_m of 0.95 μM, and low activity toward either soluble forms such as denatured collagen, isolated α chain, or isolated α1-CBl peptide, or precipitated collagen fibrils after pepsin treatment. These results indicate that the biosynthesis of the aldehyde crosslink intermediate probably occurs primarily after the onset of fibril formation in vivo. Biosynthesis of aldehydes and subsequent crosslinks may be related to the rate of fibril formation as well as to the concentration of lysyl oxidase in vivo.