Although trypsin is highly specific for lysyl and arginyl bonds, some peptide bonds, such as lysylproline, are generally trypsin-resistant, with rare exceptions as reported here. Trypsin cleaved a specific Lys-Pro bond in the chymotryptic peptide: Thr-Hyp-Ser-Hyp-Lys-Pro-Hyp-Thr-Pro-Lys-Pro-Thr-Hyp-Hyp-Thr-Tyr isolated from a Zea mays hydroxyproline-rich glycoprotein (HRGP). The daughter peptides, Thr-Hyp-Ser-Hyp-Lys-Pro-Hyp-Thr-Pro-Lys and Pro-Thr-Hyp-Hyp-Thr-Tyr, show cleavage of only one of the two Lys-Pro bonds in the parent peptide. From these and other data we suggest that there are two prerequisites for Lys-Pro cleavage: First, an extended helix characteristically present in proline or hydroxyproline-rich proteins; second, flexibility in two residues flanking the Lys-Pro bond.