Interferon- (IFN-) ± tumor necrosis factor- (TNF-) induces antiproliferation and intracellular Ca²⁺ store depletion in a human submandibular ductal cell line (HSG), which can be reversed on cytokine removal [A. J. Wu, G. C. Chen, B. J. Baum, and I. S. Ambudkar. Am. J. Physiol. 270 (Cell Physiol. 39): C514-C521, 1996]. Here we have examined a possible mechanism for the IFN--induced intracellular Ca²⁺ store depletion. There was a time-dependent decrease in thapsigargin-dependent internal Ca²⁺ release after exposure of the cells to the cytokines. The intracellular Ca²⁺ pump [sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA)] protein in lysates and membranes of cells treated with IFN- ± TNF-, but not with TNF- alone, showed a similar time-dependent decrease (examined using a SERCA2 antibody). Removal of the cytokines, which resulted in recovery of cell growth and refill of internal Ca²⁺ stores, also increased the level of SERCA protein. The decrease in SERCA is not a result of decreased cell proliferation, since thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone, or serum-free growth conditions induced antiproliferative effects on HSG cells without any corresponding decrease in SERCA. We suggest that the IFN--induced decrease in the level of SERCA accounts for the depleted state of internal Ca²⁺ stores in cytokine-treated HSG cells. These data suggest a novel mechanism for the inhibition of HSG cell growth by IFN-.