PURPOSE Cells undergoing apoptosis contribute to the regulation of activated mononuclear cells (Voll et al. 1997). Low-dose radiotherapy (LD-RT) is known to improve inflammatory symptoms, but the mechanism of action is still unclear. The aim of this study was to investigate the rate of apoptosis of peripheral blood mononuclear cells (PBMC) induced by LD-RT within the therapeutic dose range of anti-inflammatory RT. MATERIALS AND METHODS PBMC were isolated from venous blood of ten healthy volunteers and were irradiated with single doses between 0.1 and 3.0 Gy. Apoptotic nuclei were detected by flow cytometry after propidium iodide (PI) triton staining, and apoptotic cells were detected by annexin V/PI staining and cell scatter analysis. Since apoptotic cells display increased cytoplasmatic granularity and concomitant reduced cell size, they can be distinguished from viable cells in forward/side scatter (FSC/SSC) histograms. Apoptotic PBMC were further subtyped by double staining with annexin V and directly labelled monoclonal antibodies recognizing the lineage-specific surface markers CD4, CD8, and CD19, respectively. The apoptosis rate of irradiated cells was analysed in a time and dose dependent fashion and was compared to a sham-irradiated control. RESULTS After irradiation, a dose-dependent increase in apoptosis was observed, with a discontinuity (plateau or peak) between 0.3Gy and 0.7Gy in 9/10 donors (90%) and 59/80 samples (74%). 8/10 donors (80%) and 38/80 samples (47%) showed not only a discontinuous increase with a plateau but a relative maximum of apoptosis peaking within the dose range of 0.3 Gy and up to 0.7 Gy. CONCLUSION LD-RT induces a relative maximum of apoptosis in PBMC in the does range between 0.3 Gy and 0.7 Gy. This may contribute to its anti-inflammatory effect observed clinically.