Understanding the biological interactions of graphene-based materials is important for the safe use of these materials. Previous studies have explored the interaction between graphene oxide (GO) and macrophages but not the impact of GO on neutrophils, key cells of the immune system. Here, we synthesized GO sheets with differing lateral dimensions and showed by using an array of analytical and imaging techniques, including transmission and scanning electron microscopy, confocal microscopy, and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), that GO elicited the formation of neutrophil extracellular traps (NETs). ToF-SIMS revealed pronounced perturbations of plasma membrane lipids, including a decrease in cholesterol and increased levels of oxidized cholesterol species. The induction of NETs was size dependent and associated with the production of mitochondrial reactive oxygen species and calcium influx. Importantly, antioxidant treatment reduced the production of NETs. These studies provide evidence that a previously undescribed biological effect of GO manifests through direct effects on membrane lipids. Graphene oxide (GO) is being investigated for various biomedical applications. Understanding the interactions between GO and living cells is of critical importance for the safe use of these materials in patients. In the present study, we identified effects of GO on neutrophils, the most common type of white blood cell. We first synthesized GO sheets of different sizes and carefully characterized the materials. Then, using various analytical and imaging techniques, we found that GO triggered so-called neutrophil extracellular traps or NETs. NETs are normally deployed by neutrophils to capture and destroy pathogens. We were able to show that GO caused significant changes in the lipid composition of the neutrophil cell membrane, whereby the oxidation of cholesterol set into motion a cascade of intracellular events leading to the formation of NETs. These studies show that GO acts directly on the neutrophil cell membrane and leads to the activation of a conserved anti-pathogen response. Graphene oxide (GO) is a promising material for a variety of biomedical and other applications. The increasing use of GO necessitates careful assessment of potential health hazards. Using primary neutrophils as a model, Mukherjee et al. show that GO elicits neutrophil extracellular traps. Furthermore, by using ToF-SIMS, the authors noted pronounced perturbations of plasma membrane lipids in cells exposed to GO.