A protein-engineered beta-lactamase (N132 D with enhanced selectivity to 6-APA was used to construct a whole-cell immersible biosensor for 6-APA based on the immobilization of beta-lactamase-rich cells of Escherichia coli on a flat pH electrode. The response time was between 3.5-7 min. The sensitivity for penicillin was two to five fold lower to that for 6-APA. Selectivity of 6-APA was maximum at pH 7.6 using a 0.05 M phosphate buffer. 6-APA in concentrations 1mg to ml(-1) could be detected. At least 70 analyses could be performed over II days with no loss of sensitivity.