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Neutrophils play a crucial role in host defense against infectious disease. The objective was to analyze the effect of omeprazole treatment on indexes of neutrophil function in healthy subjects.Open.University hospital.Ten healthy subjects.Analysis of blood samples before and after omeprazole administration.Neutrophil Escherichia coli phagocytosis was assessed by microscopy and flow cytometry. Intracellular production of reactive oxygen intermediates was measured by flow cytometry. Extracellular reactive oxygen intermediate production was assessed with a cytochrome c reduction assay. Neutrophil bactericidal capacity and intracellular concentrations of Ca2+ were determined by fluorometry. Four hours after a single 40-mg dose of omeprazole, intra- and extracellular reactive oxygen intermediate production by neutrophils was significantly reduced compared with pretreatment values: −30% (24% to 42%) (median and range) and −22% (21% to 68%;p < .05 for both). The intracellular Ca2+ concentrations in resting neutrophils were significantly increased (+33%, 21% to 39%, compared with pretreatment concentrations, p < .001) and neutrophilic bactericidal activity was decreased (−30%, 19% to 47%, compared with pretreatment concentrations, p < .0001). Intracellular Ca2+ concentrations correlated with intracellular reactive oxygen intermediate production and neutrophilic bactericidal capacity (r = .730 and r = .618, p < .05 for both, respectively). In contrast, phagocytosis rates were not impaired by omeprazole.Our results imply that omeprazole impairs production of reactive oxygen intermediates by neutrophils. Whether specific impairments of neutrophil host defenses occur in vivo remains uncertain. Reduced bactericidal activity is associated with an increase of intracellular Ca2+ concentrations in resting neutrophils.