Ionizing radiation-induced DCF fluorescence as determined by flow cytometry for four different experimental methods. The number of cells per fluorescent bin is plotted for sham-irradiated cells (○) and 7-Gy irradiated cells (•). Panel A: Attached cells were incubated with DCFH-DA for 30 min, washed, collected, irradiated and assayed by FACS. Panel B: Same as panel A except cells when collected were centrifuged at weaker force. Panel C: Attached cells were incubated with DCFH-DA for 30 min, irradiated, washed, collected and assayed by FACS. Panel D: Attached cells were washed, collected, incubated with DCFH-DA for 30 min, irradiated and assayed by FACS. The details of each protocol are given in the Materials and Methods