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Abstract Aim: Sustained cell shrinkage is associated with apoptosis. Apoptotic volume decrease, which is known to be induced by release of osmolytes including Cl− ions, may be an essential event for apoptosis induction. Provided any anion channels and/or anion transporters are basally functioning, there is a possibility that imposition of a driving force for Cl− efflux per se results in sustained cell shrinkage and thereby induces apoptotic death. Here, this possibility was tested by reducing the extracellular Cl− concentration. Methods: Human lymphoid U937 and epithelial HeLa cells were provided for experiments after exposing to isotonic electrolyte solution which contains 146 or 1 mmCl−. Measurements of mean cell volume, caspase-3 activity and cell viability were performed by a Coulter-type cell size analyzer, a fluorometric assay and a colorimetric assay, respectively. Results: After exposure to low Cl− solution in which most chloride was replaced with aspartate, gluconate, phosphate or methanesulphonate, both U937 and HeLa cells exhibited, for up to 60 min, shrinkage to a level (90–80%) significantly lower than that in control high Cl− solution. Reduction in cell viability started within 2 h and reached below 20% within 8 h after exposure to low Cl− solution. The cell death was found to be associated with caspase-3 activation and DNA fragmentation. Conclusions: Exposure to isotonic low Cl− solution induced sustained shrinkage and thereafter apoptotic death in U937 and HeLa cells. Thus, it is suggested that sustained cell shrinkage per se provides a sufficient condition for apoptosis induction.