Cell death may occur by either of two mechanisms: necrosis or apoptosis (programmed cell death). In this paper, we demonstrate extensive chromatin cleavage into oligonucleosome-length fragments (DNA ladder) in transplanted Morris hepatoma 7777 tissue, which is suggestive of the stimulation of an endogenous endonuclease activity previously found to be involved in the process of apoptosis. The existence of many apoptotic cells, which are morphologically characterized by condensed cytoplasm and basophilic nuclear fragments, were also seen in this tissue. In vivo and in vitro experiments were designed to further differentiate the morphological and biochemical features of necrosis and apoptosis in liver and hepatoma cells. Liver tissue undergoing ischemic necrosis showed a distinct DNA ladder pattern without demonstrating the morphology of apoptosis, indicating that chromatin cleavage into oligonucleosomal-length fragments is not confined to apoptotic cell death, at least in liver cells. In in vitro-cultured McA-RH7777 cells, however, DNA ladder pattern was detected only in cells showing characteristic morphology of apoptosis. From these two criteria (i.e., characteristic morphology and DNA ladder), it was strongly suggested that the apoptotic process is highly activated in the transplanted 7777 tissue. Based on the results obtained from in vitro experiments, it was suggested that tumor apoptosis may represent a residual attempt at autoregulation within the expanding tumor population and/or may result from mild cellular injuries such as hypoxia, nutrient deficiency, or other unknown noxious factor(s). We also showed evidence that apoptosis is inducible in hepatoma cells in vitro by a wide range of mild injuries or stimuli.