Abstract. The ability of glucocorticoids to induce apoptosis or programmed cell death in mouse thymocytes is well-established. Measurement of apoptosis-associated internucleosomal DNA fragmentation through determination of the percentage of fragmented DNA by electrophoresis or centrifugation of whole cell lysates is by far the most common means of quantifying apoptosis. Since these methods measure DNA fragmentation in whole cell lysates rather than intact cells, they have severe limitations, particularly with heterogeneous cell populations. When mouse thymocytes were incubated with glucocorticoids, fixed, stained with propidium iodide and analysed flow cytometrically for cell cycle distribution, a distinct subpopulation of cells was observed to form below the Go/G1 region, denoted as the A0 region. The presence of cells in this region was consistent with the presence of internucleosomal DNA fragments as determined by gel electrophoresis. Inhibitors of transcription, translation and endonuclease activity, and a glucocorticoid receptor antagonist prevented accumulation of cells in this region. Irradiation of mouse thymocytes also produced a population in the A0 region. Cells in this region are believed to have undergone glucocorticoid-induced DNA fragmentation. This method represents a useful alternative to whole cell lysate assays, since apoptosis can be evaluated on an individual cell basis.