Thermoresponsive colloidal hydrogels (aqueous microgels) made from poly[oligo (ethylene glycol) methacrylate] (PEGMA) are an interesting class of biomaterials due to their sharp thermal transition and excellent biocompatibility. However, the inherent protein repellency of PEGMA has made the biofunctionalization of these microgels difficult and prevented them from reaching their full potential in applications such as a protein carrier. Here, we report the synthesis of thermoresponsive PEGMA microgels and the covalent attachment of horseradish peroxidase (HRP, as a model protein) to these microgels. We prepared our microgels by the precipitation copolymerization of 70 mol % OEGMA(300) and 30 mol % methacrylic acid in water. The resulting microgels showed a volume phase transition temperature (VPTT) of ca. 60 degrees C in acidic buffers and in DI water but in neutral and basic buffers did not show a thermal response up to 75 degrees C. The direct immobilization of HRP to the activated carboxylic groups in the microgel backbone was unsuccessful, but using a diamine spacer with four ethylene glycol units, we were able to covalently attach this enzyme to our PEGMA microgels through bis-aryl hydrazone chemistry. HRP has its maximum activity at ca. 50 degrees C. At higher temperatures, the activity was reduced, but the microgel-bound enzyme showed less reduction in activity than the native enzyme and no change in activity associated with the VPTT. In addition, PEGMA microgels stabilized the attached enzymes against thermal denaturation. For example, our results showed that the enzyme immobilized on the PEGMA microgel lost its activity 3.4 times slower than the free enzyme in the first 5 h of annealing at 50 degrees C. The bioconjugation strategy introduced here could serve as a model for the covalent attachment of other biomacromolecules to the protein-repellent PEGMA microgels.