We have developed a reusable piezoelectric immunosensor for human granulocytes. Three different methods for immobilization of the anti-CD 18 antibody on the gold electrode of the immunosensor were tested. The methods showed some differences in terms of stability and sensitivity. Applying an anti-CD 18 antibody layer onto a 10-MHz AT-cut crystal resulted for 2 × 103to 3 × 105cells on the surface in a linear frequency change. The modified crystal was stable for eight weeks when stored desiccated at room temperature.