OCI/AML-2 acute myeloid leukemia cells were found to undergo apoptosis after treatment with y rays from a 137Cs source. Multilaser flow cytometry techniques using probes for live cell function were used to monitor the biochemical changes that occurred prior to the loss of surface membrane integrity. These showed increases in the generation of reactive oxygen species (ROS) and in the glutathione (GSH) content of irradiated cells. An additional population of cells that showed a further increase in ROS and depletion of GSH was seen in irradiated cells but not in controls. This population showed loss of mitochondrial membrane potential (deltapsim), indicative of the mitochondrial permeability transition, and exposure of phosphatidylserine on the cell surface. Increases in intracellular calcium were observed in a proportion of these low-deltapsi(m)/high-ROS cells. Similar findings were seen using the antileukemia drug cytosine arabinoside (ara-C), although cell cycle analysis showed that the loss of deltapsi(m) occurred mainly in G1 phase with ara-C treatment, and mainly in G2 phase with irradiation. Furthermore, the protective effect of overexpression of BCL2 was more pronounced after ara-C treatment than with radiation. Cells of the TP53 (formerly known as p53)-null human AML line OCI M2 showed growth arrest in G2 phase after radiation treatment, with no loss of deltapsi(m) or morphological changes indicative of apoptosis. The flavine-dependent oxidoreductase inhibitor diphenylene iodonium failed to inhibit generation of ROS in irradiated OCI/AML-2 cells, indicating that the mechanism is unlikely to involve the TP53-induced gene PIG3. These results show that oxidative stress can occur in irradiated human leukemia "blasts", and may play a direct role in radiation-induced apoptosis.