A significant decrease of sister chromatid exchange (SCE) frequencies was observed in Bloom's syndrome(BS) fibroblasts by cocultivation with normal fibroblasts at 5:1, 1:1, 1:5 ratios, and by culture in medium previously used to support normal fibroblasts. On the other hand, the SCE rate of normal fibroblasts was not modified. These results are compared with those from other studies and discussed.