Natural Killer cells are immune cells that recognize and eliminate altered and non-self cells from the circulation. To study the interaction between NK cells and target cells, we set up an experimental system consisting of rat Interleukin-2 activated Natural Killer cells (A-NK cells) and rat hepatocytes with a masked Major Histocompatibility Complex (MHC). The masking of the MHC induces recognition of the hepatocytes by the NK cells as non-self. We showed that in vitro apoptosis is rapidly induced in the hepatocytes [Blom et al., 1999] after co-incubation with A-NK cells. Now we describe the morphological changes that occur during and after interaction of A-NK cells with hepatocytes. Confocal laser scanning microscopy showed that the actin cytoskeleton of the NK cells was remodeled during attack of hepatocytes. Some NK cells were in close contact with the hepatocytes while others had formed actin-containing dendrites of varying length that made contact with the hepatocytes. However, dendrite formation is not obligatory for induction of apoptosis because cells that were unable to form these did induce FAS-dependent apoptosis in hepatocytes. Apparently both direct as well as distant contact resulted in apoptosis. Formation of the dendrites was calcium-dependent as EGTA largely prevented it. Importantly, chelation of the calcium also suppressed killing of the hepatocytes. Within 1 h after addition of the A-NK cells, morphological changes in hepatocytes that are characteristic of apoptosis, such as the formation of apoptotic bodies and fragmented nuclei, became apparent. Specifically, the actin cytoskeleton of the hepatocytes was remodeled resulting in the formation of the apoptotic bodies. Inhibition of caspase activity by z-Val-Ala-DL-Asp-fluoromethylketone (100 microM) partly protected against the rearrangement of the actin filaments in the hepatocytes.