The aim of the present study was to assess the potential of the test substance isobutyraldehyde or its metabolite(s) to induce structural chromosomal aberrations. For this purpose, an in vitro cytogenetic assay was carried out to determine chromosome aberration frequencies in V79 cells. Most of the substances are not mutagenic or carcinogenic themselves, but only after metabolic transformation, which in vivo is catalyzed mainly by the enzyme systems of the liver. Therefore, the tests are carried out not only directly, but also in the presence of an exogenous metabolic activation system. The most commonly used system is a cofactor- supplemented postmitochondrial fraction (S-9) obtained from livers of rats treated with an enzyme-inducing agent, e.g. Aroclor 1254.