Resulting from its subjective nature, cytopathic effect (CPE) due to virus infection in cell culture has long been difficult to quantify. This report illustrates the use of electric cell-substrate impedance sensing (ECIS) for monitoring the progression of CPE due to influenza A virus infection. ECIS monitors the impedance of a non-invasive ac current flowing through cell culture medium by gold film electrodes placed on the surface of the culture dish. As cultured cells attach and spread onto the electrodes, the current is impeded proportional to the number of attached cells, the number of tight junctions between cells and the shortness in distance between the cells and the substratum. In the case at hand, a healthy monolayer of cells was insulted with influenza A virus infection and exhibited a characteristic rounded cell morphology and cell detachment. These effects resulted in reduced impedance, which was monitored with ECIS. Since data obtained through ECIS are both quantitative and in real-time, it was possible to monitor continuously cell behavior during infection. This, in turn, allowed for a more detailed and comprehensive data set to analyze. More importantly, through ammonium chloride treatment of cells, it was also shown that ECIS may be exploited to examine a treatment's effect on the reduction of resistance because of its antiviral activity. Thus, ECIS may be a powerful approach for screening antiviral compounds quantitatively in a real-time fashion.