Chinese hamster cells in vitro were double labeled with C(14)TdR and H(3)TdR. At the time of irradiation with Co(60) gamma rays (600 rad), the cells in the G(2) phase were labeled only with C(14), whereas cells in the late and middle S phases were labeled with both C(14) and H(3). The cells in early S phase were labeled only with H(3) and the G(1) 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 G(1) cells divided prior to some of the cells which were in the S phase denoting mixing of the populations. The G(2) phase sustained three times more chromosomal damage than the S phase. Little difference in chromosomal damage was found between the G(1) and S phases or among the different parts of the S phase. Cells in G(2) 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 G(1) cells but only chromatid exchanges were induced in S and G(2) 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.