Natural killer (NK) cells play a crucial role in the elimination of virus-infected or transformed cells in the liver. In this article, we describe the mechanism by which liver cells are killed by NK cells. Interleukin-2-activated natural killer (A-NK) cells from the rat induced apoptotic cell death in 30% of freshly isolated rat hepatocytes within 60 minutes. Recognition by the A-NK cells of the hepatocytes as nonself was established by masking the major histocompatibility complex (MHC) class I molecules on the hepatocytes with the OX18 antibody. During the killing process, a decrease of the mitochondrial membrane potential (MMP), formation of blebs, phosphatidyl serine (PS) externalization, chromatin condensation, and nuclear fragmentation were observed. The hepatocytes became apoptotic before permeabilization of the plasma membrane occurred, suggesting that the observed cytolysis was caused by secondary necrosis. The apoptotic process was completely abolished by the caspase inhibitors, Z-Val-Ala-DL-Asp fluormethylketone (zVAD-fmk) and Ac-Asp-Glu-Val-aldehyde (DEVD-cho). However, under these conditions, A-NK cells killed a smaller fraction of the hepatocytes by (primary) necrosis. These results indicate that apoptosis is the major cytotoxic process induced by A-NK cells in hepatocytes. If apoptosis is prevented, a more limited necrotic effect is induced. Therefore, this study shows that NK cells are fully equipped to induce both apoptosis and necrosis in hepatocytes, but appear to prefer the apoptotic route.