A delay in cell cycle progression and a decrease in sister chromatid exchange (SCE) frequency was observed in Bloom syndrome (BS) cells when exposed to polyethylene glycol (PEG) treatment followed by co-cultivation with unexposed normal cells, whereas the SCE rate of both normal and BS cells increased when PEG-exposed normal cells were co-cultured with unexposed BS high SCE cells. This indicates the role of normal cells, probably of cell membranes (which were disturbed or disrupted by PEG treatment), in complementing the deficiency of BS cells under co-culture conditions, and suggests that BS cells may be deficient in membrane bound factor(s) present in normal cells.