Cross-linking chemical modification of pyrroloquinoline quinone (PQQ) glucose dehydrogenase (GDH) by glutaraldehyde was carried out and its stability was analyzed. Although native PQQGDH was inactivated within 30 min at a higher temperature than 50 degrees C cross-linked PQQGDH retained more than 40% of initial activity even after 30 min of incubation at 54 degrees C. In addition to the increase in thermal stability, cross-linked PQQGDH gained high EDTA tolerance. The stabilization may be achieved by increased the rigidity of PQQGDH hole enzyme conformation.