Sister chromatid exchanges (SCE's) which are easily seen by "harlequin chromosome" techniques can be readily induced in cultured Chinese hamster ovary (CHO) cells by low concentrations of mutagen-carcinogens that do not require metabolic activation. If the cells are simultaneously treated with cyclophosphamide which does require metabolic activation before it becomes mutagenic, and an activating system consisting of an extract of rat liver containing microsomes (S-9 Mix) then numerous SCE's are induced by the compound. This indicates that the induction of sister chromatid exchanges in such cells can be used as an in vitro assay for mutagens that require activation as well as those that do not. The method, which is very simple and quick, is more sensitive than is the usual cytogenetic assay in which chromosome aberrations are assayed.
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