Anaplasma phagocytophilum is a Gram-negative obligate intracellular bacterium that is transmitted by ticks of the Ixodes ricinus complex. It replicates in neutrophils and elicits febrile disease in humans and animals. Because of its striking tropism for neutrophils, A. phagocytophilum has been used as a model organism to study the immune response against obligate intracellular pathogens. In mice, the control of A. phagocytophilum in the early phase of infection is dependent on natural killer cell-derived interferon- (IFN-). In contrast, the final elimination strictly requires CD4(+) T-cells. It is a matter of debate, whether neutrophils serve only as host cells or as killer cells as well. To study this, we used in vitro generated murine neutrophils with defects in major antimicrobial molecules such as NADPH-oxidase (gp91(phox-/-)), myeloperoxidase (MPO(-/-)) and inducible nitric oxide synthase (iNOS(-/-)). However, bacterial growth in gene-deficient neutrophils was comparable to that in wild-type cells. Whereas gp91(phox) and MPO expression remained unchanged, the infection led to an induction of iNOS. In neutrophils stimulated with IFN-, bacterial growth was significantly impaired, and iNOS was induced. However, the antibacterial effect of IFN- was still seen in iNOS(-/-) neutrophils. Thus, murine in vitro generated neutrophils stimulated with IFN- seem to act as killer cells by an iNOS-independent mechanism.