The potentiometric alternating biosensor (PAB) system has been utilized to monitor the effects on normal versus cancer cells of Mitoxantrone, a potent inhibitor of nucleic acid synthesis commonly utilized in the clinical treatment of hepatocarcinomas. In this first series of toxicity tests, we have utilized a primary culture of rat hepatocytes and a stabilized line named HepG2 from a human hepatoma. Primary cultures of hepatocytes represent a unique tool in toxicology/pharmacology as shown in previous works. A suitable microvolume flow chamber has been designed and produced. The chamber is equipped with inlet and outlet circuits and with a fixed transducer (Si/SiO2/Si3N4 chip) facing a cover slip on which cells grow. The PAB system is here utilized only in the pH-sensitive configuration with a single measuring spot, warranting an accurate determination of the change in the extracellular acidifcation rate resulting from drug administration. We have also observed the changes in the acidification rate of 3T6 mouse fibroblasts, induced by a treatment with Ara-C, a well-known DNA antimetabolite. New insights can be obtained from these studies into the drug response of normal versus cancer cells and into the feasibility of real-time PAB monitoring of drug toxicity and efficacy, towards effective human cancer treatment. The effect of increasing drug concentrations on cellular metabolism is here compared with the results obtained from conventional tests (optical microscopy, Neutral Red and Trypan Blue assays).