The human neutrophil plays an important role in defending the body from infection by eliminating invading microorganisms. To fulfill its tasks, the cell stores a plethora of bactericidal proteins and mobilizable cell surface receptors in intracellular organelles, so-called granules and secretory vesicles. The neutrophil communicates with its environment by sequentially mobilizing these organelles to the plasma membrane. The most easily mobilized organelles are the socalled secretory vesicles followed by the gelatinase granules, and mobilization of these is crucial for neutrophil interaction with the endothelium and migration into the tissue [1, 2]. Subsequently, the specific and azurophil granules are mobilized, a process of importance for phagocytosis and killing of microbes by fusing primarily with the phagosome [3]. Little is known about the underlying mechanisms that mediate docking and fusion of the granules/vesicles in neutrophils [4], but their release is known to be Ca21 dependent [3]. The azurophil granules have traditionally been regarded as the neutrophil counterpart to lysosomes, but since they lack the lysosomal marker lysosome-associated membrane glycoprotein (LAMP) they have lately been suggested to be non-lysosomal secretory organelles [5, 6]. The membrane of the azurophil granule is poorly characterized. Until a few years ago, CD63 and CD68 were the only azurophil granule membrane proteins identified [7–9]. Using MALDI-TOF MS of tryptic peptides from isolated azurophil granule membranes, we recently identified a new azurophil granule membrane component, stomatin [10]. We also showed the presence of detergent-insoluble, low-density membrane domains, so-called detergent-resistant domains (DRMs) in