Chinese hamster cells in vitro were double labeled with C14TdR and H3TdR. At the time of irradiation with Co60 gamma rays (600 rad), the cells in the G2 phase were labeled only with C14, whereas cells in the late and middle S phases were labeled with both C14 and H3. The cells in early S phase were labeled only with H3 and the G1 cells were unlabeled. Samples were fixed at various time intervals following irradiation and the metaphases were analyzed for chromosomal damage. The phase in which the cell was located at the time of irradiation was determined by counting grains in the first and second layers of autoradiographic film. In both control and irradiated cells some G1 cells divided prior to some of the cells which were in the S phase denoting mixing of the populations. The G2 phase sustained three times more chromosomal damage than the S phase. Little difference in chromosomal damage was found between the G1 and S phases or among the different parts of the S phase. Cells in G2 sustained a mitotic delay of 4 hr, while the other phases sustained a delay of 2 to 3 hr. Chromatid and chromosome (dicentrics) exchanges were induced in G1 cells but only chromatid exchanges were induced in S and G2 cells; this is consistent with the hypothesis that the chromosome consists of two subunits which separate either slightly before or immediately as the cell enters the S phase.