Ammonium ions induced significant apoptosis in hybridoma cells that were suspended in PBS containing glucose and glutamine; 40% of the cells became apoptotic within 200 min. In the presence of bumetanide, an inhibitor of the Na(+)K(+)2Cl(-)-cotransporter, the ammonium ion induced apoptotic response was completely abolished. This result shows that the inward transport of ammonium ions by the Na(+)K(+)2Cl(-)-cotransporter was the sole cause of apoptosis in the ammonium (and ammonia) exposed hybridoma cells.