TNF is a protein toxin which is secreted by activated macrophages and monocytes. Although the cytotoxic activity of TNF has been well documented, the mechanism of TNF-induced lysis is not well understood. The goal of this investigation was to determine whether TNF caused one of the classic forms of cell death, i.e., apoptosis, which is characterized by nuclear disintegration and cytoplasmic "boiling," or necrosis, which is characterized by the formation of a "balloon-like" plasma membrane and a lack of nuclear disintegration. Therefore, to distinguish apoptosis from necrosis, we have used time-lapse video microscopy to observe the death of several TNF-sensitive target cell lines while measuring the release of Na2(51)CrO4 and [3H]TdR from cytoplasmic and nuclear compartments, respectively. As targets we selected two spontaneously sensitive cell lines, F17 and L-M, and one resistant cell line, C3HA, which was sensitized by treatment with cycloheximide or by infection with the adeno-SV40 hybrid virus Ad2+ND2. We find that the type of cell death observed depends on the cell being tested. For example, in F17 cells we found that TNF treatment induced a classical form of apoptosis. In contrast, TNF induced a necrotic form of cell death in L-M cells, similar to the lysis induced by antibody and C. Finally, we found that sensitized C3HA cells displayed a novel cytolytic phenotype which resembled apoptosis but did not include DNA fragmentation. These results emphasize the complex nature of the TNF-induced cytotoxic response.