Flow cytometry instantaneously determines the percentages of cells in various cell cycle phases to rapidly evaluate effects of irradiation and anti-cancer drugs. We studied these using growth curve analysis and 5-bromodeoxyuridine propidium iodine (BrdU)-(PI) double staining. With conventional DNA histogram methods, determination of S phase fraction was difficult because of overlapping DNA content between G1 and early S phases and between late S and G2 phases. Double staining directly differentiated G1, S, and G2 + M phases. By double staining, rapid transition from G1 to S occurred within 4 h after irradiation or after the drug treatments, and initial G1 arrest induced by irradiation was confirmed for the first time.