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The relation of O2.−-production and Ca2+ homeostasis was investigated in PLB-985 cell lines and neutrophilic granulocytes from peripheral blood. In differentiated wild-type PLB-985 cells, a high level of O2.−-production was associated with a significant decrease in the membrane potential and the inhibition of capacitative Ca2+ entry. These correlations were not observed in gp91phox −/− cells or in cells transfected with a non-functional mutant of gp91phox (Thr341Lys). Membrane depolarization and inhibition of Ca2+ entry reappeared in cells transfected with wild-type gp91phox. These experiments demonstrate that inhibition of Ca2+ entry depends on the presence of a functional NADPH oxidase. The Ca2+ signal induced by stimulation of chemotactic receptors also showed remarkable differences: [Ca2+]ic in the sustained phase was higher in gp91phox–/– than in wild-type cells. Alteration of the Ca2+ signal was reproduced by treating peripheral blood neutrophils with the NADPH oxidase inhibitor diphenylene-iodonium. It is concluded that the deficiency in O2.−-production is accompanied by significant alterations of Ca2+ homeostasis in myeloid cells.