We recently established four lymphoblastoid cell lines (LCLs) by infecting the peripheral blood of four Japanese patients suffering from Bloom's syndrome (BS) with Epstein-Barr virus (EBV). During the course of propagating these cell lines, two of them exhibited dimorphism regarding spontaneous sister chromatid exchange (SCE), i.e., a mixed population consisted of cells with extremely high SCE levels characteristic of BS and cells with low SCE levels indistinguishable from that of normal control cells. On the other hand, the other two cell lines maintained a monomorphic population with high SCE levels at least until 30 weeks after EBV infection. The proportion of the cells with high SCE levels in the cell lines with dual phenotype declined as the population doubling numbers (PDN) increased with time and they became ultimately undetectable. The proportion of cells with low SCE levels at the time of EBV infection was estimated in one of these LCLs as 0.075% by extrapolating the linear regression of the logit for the proportion plotted against PDN. In view of the well-known stability of the monomorphic phenotype in representative BS LCLs during extended cultivation, together with the present observations on the dual phenotype, we conclude that the frequent establishment of BS LCLs exclusively with low spontaneous SCE levels is attributable to the various proportions of low-SCE cells existing in vivo in the B-lymphocytes pool of BS individuals and to the selective pressure against the high-SCE cells in in vitro cultures.