Electropermeabilization (electroporation) is a technique widely used to introduce various membrane-impermeable molecules into cells in vitro or in vivo. In this study we determined the effect of different electric-field intensities on electropermeabilization and electrosensitivity of a variety of tumor-cell lines in vitro. For this purpose we used two assays: propidium iodide uptake for measurement of cell electropenneabilization, and the clonogenic or MTT assay for determination of electrosensitivity. Our results showed that electropermeabilization of almost all cell lines tested occurred at 600 V/cm. In contrast, a marked difference in electrosensitivity existed among these cell lines. Our results could be of great importance for pharmacological and biochemical studies in vilro, and for prediction and determination of tumor response in vivo to electropermeabilization combined with chemotherapeutic drugs (electrochemotherapy) and gene therapy.