The exposure of the epidermoid cell line A431 to different concentrations of CsCl was assessed using different methodological approaches. Two different effects were detected depending upon the concentration of the agent: at low concentrations, cell modification was represented mainly by a very pronounced cell flattening and an alteration of the cell-to-cell contacts, interpreted as an increase in cell adhesion. At higher concentrations, a clear pathogenic effect was observed that allowed the formulation of the hypothesis that specific mechanisms of toxicity at the subcellular level involving mitochondrial and cytoskeletal function can exist. In addition, membrane order parameters, as detected by electron paramagnetic resonance (EPR) spectroscopy, displayed a dose-dependent increase in membrane rigidity. Results reported here seem to suggest that cesium ions can enter the cell, modify plasma membrane integrity and alter some specific cytoplasmic components, e.g. the cytoskeleton. Considering that environmental contamination by cesium as a result of radioactive fallout is of major importance and that few data are available thus far on this matter, this study provides evidence for the possible mechanisms of action of the non-radioactive form of this ion in cells.