IL-3 deprivation has been reported to induce apoptosis of bone marrow-derived mast cells. In order to evaluate this type of cell death further, we employed trypan blue and propidium iodide stainings, photometric enzyme immunoassay, fluorescence measurement of caspase-3, DNA electrophoresis, flow cytometry and transmission electron microscopy. In this experiment, although several evidences supporting apoptosis were demonstrated some findings were not consistent with typical apoptosis. On the other hand, electron microscopical observation demonstrated that most cells from all the time phases after IL-3 deprivation showed the morphology of typical oncosis, i.e. cell swelling, disintegration of ultrastructure and subsequent karyolysis. Only a small number of cells from the later time phases showed apoptotic morphology. We here suggest that BMMCs undergo both apoptosis and oncosis after IL-3 deprivation and that the dominant type of prelethal change is oncosis in all time phases, although apoptosis also plays a partial role in the late time phases.