When the CD4(+)CD8(+) thymic lymphoma cells were treated with puromycin, we found that most of the cells died at 0.3-1 microg/ml of puromycin within 24h. However, cell death was greatly reduced when the dose of puromycin was increased. Similar dose-pattern of cell death was observed in thymocytes and the sensitivity to puromycin was greater in CD4(+)CD8(+) thymocytes than CD4(+)CD8(-) thymocytes. The induction of apoptosis was blocked by the protein synthesis inhibitor cycloheximide, and to some extent by transfection of Bcl-xL or Bcl-2 genes. Expression of GRP78 was up-regulated after treatment with a small dose of puromycin, and the cell death by puromycin was blocked in the presence of caspase 12 inhibitor. These results indicated that the induction of cell death by low-dose puromycin was due to endoplasmic reticulum stress. Furthermore, we found that dexamethasone, a synthetic glucocorticoid, and puromycin worked synergistically to induce cell death in thymocytes.