AbstractNeutrophils are important effector cells in the host defense against invading microorganisms. One of the mechanisms they use to eliminate pathogens is the release of neutrophil extracellular traps (NETs). Although NET release and subsequent cell death known as NETosis have been intensively studied, the cellular components and factors determining or facilitating the formation of NETs remain incompletely understood. Using various actin polymerization and myosin II modulators on neutrophils from healthy individuals, we show that intact F-actin dynamics and myosin II function are essential for NET formation when induced by different stimuli; that is, phorbol 12-myristate 13-acetate, monosodium urate crystals, and Candida albicans. The role of actin polymerization in NET formation could not be explained by the lack of reactive oxygen species production or granule release, which were normal or enhanced under the given conditions. Neutrophils from patients with very rare inherited actin polymerization defects by either actin-related protein 2/3 complex subunit 1B or megakaryoblastic leukemia 1 deficiency also failed to show NETosis. We found that upon inhibition of actin dynamics, there is a lack of translocation of neutrophil elastase to the nucleus, which may explain the impaired NET formation. Collectively, our data show the essential requirement of an intact and active actin polymerization process, as well as active myosin II to enable the release of nuclear DNA by neutrophils during NET formation.