Fusion of fresh lymphocytes from a Bloom syndrome (BS) patient with those of normal subjects or a BS heterozygote resulted in complete normalization of the frequency of sister chromatid exchanges in the chromosomes of BS cells. This normalization took place by the first mitosis in hybrid cells. In contrast, cultivation of BS lymphocytes with those of normal subjects or the BS heterozygote had no effect on sister chromatid exchanges. The cell fusion experiments suggest that the normalization on the sister chromatid exchanges. The cell fusion experiments suggest that the normalization of the sister chromatid exchange frequencies in BS cells can be achieved by factors conserved in the cells of various mammalian species. These findings are compatible with the concept that BS is a recessive genetic mutation at regulatory levels of the DNA repair function.