The sister chromatid exchange (SCE) assay is a short-term test for the detection of reciprocal exchanges of DNA between two sister chromatids of a duplicating chromosome. Detection of SCEs requires some means of differentially labeling sister chromatids, which can be achieved e.g. by incorporation of bromodeoxyuridine (BrdU) into chromosomal DNA for two cell cycles. SCEs can be measured in mammals as well as in non-mammalian systems. The test substance should be solid, liquid, vapour or gaseous. Mammalian cells in vitro are exposed to the test chemical with and without an exogenous mammalian metabolic activation system, and cultured for two rounds of replication in BrdU containing medium. At least three adequately spaced concentrations of the test substance should be used. After treatment with a spindle inhibitor to accumulate cells in a metaphase-like stage of mitosis (c-metaphase), cells are harvested and chromosome preparations are made.