Repeated measurement of spontaneous and clastogen-induced sister-chromatid exchange.

Sister-chromatid exchanges (SCE), both spontaneous and chemically-induced [bleomycin (BLM), mitomycin-C (MMC), streptonigrin (SN), and 4-nitroquinoline-1-oxide (4NQO)], were studied in the lymphocytes of 24 normal individuals on 2 or 3 different occasions, separated by periods of up to 2 years. For all BLM-induced SCEs, the variation in SCE frequency among the samples from a single individual was significantly greater than the variation between replicate cultures on a given day. These results raise questions concerning the validity of conclusions based on a single observation of chemically-induced SCEs.