Apoptosis is a distinct mechanism by which eukaryotic cells die. Neutrophils (PMN) play a fundamental role in the systemic inflammatory response syndrome. Clearance of PMN during resolution of the acute inflammatory process occurs by apoptosis, but factors inducing this process are unknown. The aims of this study were to determine whether PMN ingestion of Escherichia coli would result in PMN apoptosis and whether the mechanism was related to the respiratory burst, PMN from 10 healthy volunteers were cultured with different ratios of PMN:E. coli (1:0 to 1:25) for 12 h. Apoptosis was then assessed by propidium iodide DNA staining, morphology, gel electrophoresis, and Fc gamma RIII expression. There was a significant induction of PMN apoptosis on incubation with E. coli at a ratio of 1:10 and 1:25 PMN:E. coli as well as decreases in Fc gamma RIII. This correlated with increased ingestion of FITC-labeled E. coli and intracellular reactive oxygen intermediates after a 2-h coculture. To clarify the role of reactive oxygen intermediates in E. coli-induced PMN apoptosis, we assessed the effects of the antioxidants catalase, DMSO, glutathione, and N-acetylcysteine. There was a significant decrease in E. coli-induced PMN apoptosis on incubation with DMSO (1.0%), glutathione (25 mM), and N-acetylcysteine (25 mM) compared with control PMN:E. coli. This study demonstrates for the first time that E. coli induces PMN apoptosis through an oxygen-dependent mechanism. The removal of effete PMN by the process of apoptosis rather than necrosis may be teleologically beneficial during Cram-negative septicemia.Apoptosis is a distinct mechanism by which eukaryotic cells die. Neutrophils (PMN) play a fundamental role in the systemic inflammatory response syndrome. Clearance of PMN during resolution of the acute inflammatory process occurs by apoptosis, but factors inducing this process are unknown. The aims of this study were to determine whether PMN ingestion of Escherichia coli would result in PMN apoptosis and whether the mechanism was related to the respiratory burst, PMN from 10 healthy volunteers were cultured with different ratios of PMN:E. coli (1:0 to 1:25) for 12 h. Apoptosis was then assessed by propidium iodide DNA staining, morphology, gel electrophoresis, and Fc gamma RIII expression. There was a significant induction of PMN apoptosis on incubation with E. coli at a ratio of 1:10 and 1:25 PMN:E. coli as well as decreases in Fc gamma RIII. This correlated with increased ingestion of FITC-labeled E. coli and intracellular reactive oxygen intermediates after a 2-h coculture. To clarify the role of reactive oxygen intermediates in E. coli-induced PMN apoptosis, we assessed the effects of the antioxidants catalase, DMSO, glutathione, and N-acetylcysteine. There was a significant decrease in E. coli-induced PMN apoptosis on incubation with DMSO (1.0%), glutathione (25 mM), and N-acetylcysteine (25 mM) compared with control PMN:E. coli. This study demonstrates for the first time that E. coli induces PMN apoptosis through an oxygen-dependent mechanism. The removal of effete PMN by the process of apoptosis rather than necrosis may be teleologically beneficial during Cram-negative septicemia.