A novel immunosensing device based on a piezoelectric sensor for direct detection of the biological warfare agent Francisella tularensis was developed. This sensor includes mouse polyclonal antibody immobilized in a layer of protein A covalently linked to the gold electrode of the sensor. The immunosensor is able to detect F. tularensis with the limit of detection 10(5) CFU/mL with a typical measuring cycle > 5 min. The sensor was successfully evaluated for rapid detection of F. tularensis spikes in drinking water and milk; no deterioration of sensitivity in comparison with buffer solutions was observed. The proposed concept of a rapid measurement of microbial agents seems to be promising for evaluation of samples after short pre-cultivation enrichment.