Cells in third mitosis treated during the first cell cycle with 3H-TdR and during the next two cycles with BrdU (without 3H-TdR) show a typical pattern of chromosome differentiation which allows identification of sister chromatid exchanges occurring during the first (SCE1), second (SCE2) and third cycles (SCE3). Chromosomes labeled only with 3H-TdR had the most SCEs; those labeled only with BrdU, the second highest number; and those labeled with 3H-TdR plus BrdU, the fewest. Since BrdU and 3H-TdR are well known inducers of SCEs, the relatively low frequency of exchanges produced by the combined action of these two compounds is paradoxical. It is assumed that SCEs are generated by the abnormal recombination of double-strand DNA breaks occurring at the junctions between completely and partially duplicated replicon clusters. Thus, agents that induce absolute blocks to DNA fork displacement will favor the appearance of SCEs because double-strand breaks have more time to occur at junctions. Conversely, agents that inhibit the initiation of replication will decrease the probability of SCEs. Ionizing radiation delays the onset of cluster replication. Therefore, in 3H-TdR plus BrdU-substituted chromosomes the radiation from tritium may inhibit the appearance of BrdU-induced SCEs. Since the inhibition does not exist in chromosomes substituted only with BrdU, the frequency of SCEs in these elements is higher than in double-substituted chromosomes. During the first cell cycle the onset of cluster replication is normal. However, the incorporation of 3H-TdR in the replication fork may enhance the appearance of double-strand breaks, thus inducing a high frequency of SCEs.