The morphological changes in mastocytoma cell death by complement, T cell mediated cytotoxicity and in natural cell death as seen by time-lapse microcinematography are recorded. In complement lysis, progressive changes leading to a final swelling of the cell occur in the absence of cell membrane activity. This is in marked contrast to the T-cell lysis in which violent zeiosis (boiling) of the cytoplasm is a constant feature. Similar changes are seen in natural cell death and the possibility that a similar mechanism is involved in both cases is discussed. Analysis of the data showed that there was no fixed time relation between T cell contact and zeiosis. Target cells were sometimes killed rapidly after contact, but in some cases periods of up to 200 min were observed between contact and killing. After contact and up to the onset of zeiosis target cells retain normal morphology and movement, suggesting that contact itself does not damage the target cell. This is consistent with the data showing that the release of both small and large cytoplasmic markers have similar kinetics. An analysis of the relation between cell volume and time between T cell contact and zeiosis suggested that mastocytoma cells may be more susceptible to c. m. c. during G 1 phase. Studies with peritoneal exudate cells as target cells have allowed the morphological changes to be seen in a variety of other cell types. Macrophages rapidly retract to a spherical form immediately before zeiosis, indicating a change preceding zeiosis. The phase of zeiosis varies with different cell types.