$eta$-Lactam antibiotics such as penicillin G, amoxicillin, and ampicillin were determined by a potentiometric biosensor system which exploited penicillinase immobilized on Immobilon cellulose nitrate membrane and a flat-bottomed pH electrode-as the biological component and transducer. The optimum reaction buffer for maximum sensitivity was found as 2 mM of sodium phosphate buffer (pH 7.2). The detection limit of the biosensor could be extended to 1 $mu{M}$ of the analytes by increasing the enzyme loading for immobilization to 100 units/$mell$. The model samples spiked with each of the standard penicillins were measured for their biosensor responses and HPLC peak area, resulting in the relative responses of 82.1-103.5% and 79.5-106.1% for the biosensor method along with HPLC analysis, respectively. This result showed a good precision of the current biosensor method for screening the penicillin compounds.