SUMMARYNK cells represent one of the first lines of defence in the immune reaction after invasion ofLeishmaniaparasites. Depletion of mouse natural killer (NK) cells dramatically enhances susceptibility of normally resistant mice. In this study we evaluated the fate of NK cells and parasites after contact formation. The hydrophilic fluorescent dye CMFDA (chloro-methylfluorescin diacetate) that allows analysis of cytotoxicity in flow cytometry and microscopy was used. Furthermore, these findings were confirmed with scanning and transmission electron microscopy. Direct contact points were found betweenLeishmaniapromastigotes and naïve human NK cells. These contacts were associated with transfer of cytosol by membrane bridges and cytotoxicity of NK cells againstLeishmania. However, in contrast to other target cells which allow repeated exocytosis of lytic granules, contact withLeishmaniacauses immediate destruction of NK cells in a non-apoptotic way. Our results give a reasonable explanation forex vivoobservations of reduced NK cell numbers and impaired NK response in patients with acute cutaneous leishmaniasis. Animal models have clearly shown that NK cells play a key role in the induction and direction of the immune response. Thus inhibition of NK cells at the onset of infection would be advantageous for the survival of the parasite.