When Bloom syndrome (BS) cells labeled with bromodeoxyuridine (BrdUrd) for one round of DNA replication were fused with nonlabeled normal cells, the hybrid cells had a normal level of sister chromatid exchange (SCE) at the first mitosis after fusion. However, when normal cells treated with mitomycin C (MC) were fused with nontreated normal cells, the MC-induced SCE was not affected by fusion with normal cells. Single and twin SCEs were analyzed in the Colcemid-induced endoreduplicated normal and BS lymphoid B cells from diplochromosomes. In normal cells, the same number of SCEs occurs in each of the two cell cycles; the SCE ratio of single (6.30 SCEs per cell) to twin (2.92 SCEs per cell) was 2:1 on the endoreduplicated-cell basis, showing 1:1 on the diploid-cell basis. In BS cells, the SCE ratio of single (144.8 SCEs per cell) to twin (5.9 SCEs per cell) was 25:1 on the endoreduplicated-cell basis and was 12:1 on the diploid-cell basis. These studies strongly suggest that most of the BS SCEs occur during the second cell cycle when BrdUrd-containing DNA is used as template for replication and that the normal level of BS SCE observed at the first mitosis of the hybrid cells is the result of SCE inhibition resulting from the fusion with normal cells.