Increase in intracellular calcium concentrations ([Ca2+]i) is critical for the initiation of apoptosis in cells such as thymocytes and in other cells, calcium chelators may promote apoptosis. However, calcium modulators, such as calcium ionophore 4-bromo-calcium ionophore (Br-A23187) and thapsigargin (TG), induce apoptosis in different cells, including HL-60 cells in which the induction of apoptosis seems a calcium-independent process. These observations imply that the disturbance of calcium homeostasis is probably the most important factor in the regulation of apoptosis. In this article, reagents with different potencies of modulating calcium homeostasis were used to study the possible role of [Ca2+]i and the status of intracellular calcium stores in the causation of HL-60 cell apoptosis. We found that an increase in [Ca2+]i alone did not result in apoptosis, while the depletion of TG-sensitive calcium stores in the endoplasmic reticulum was closely related with the induction of apoptosis. In HL-60 cells, extracellular and intracellular calcium chelators promoted apoptosis. Calmodulin antagonist did not attenuate apoptosis induced by other reagents. Our results suggest that the depletion of Ca2+ stores is an important mean to modulate calcium homeostasis and that the mobilization of calcium (Ca2+) from intracellular stores, rather than an increase in [Ca2+]i, provides the signal for the induction of apoptosis in HL-60 cells.