The CHO/HPRT test detects forward mutations of the X-linked hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus in Chinese hamster ovary (CHO) cells (1, 2). HPRT catalyzes not only the conversion of hypoxanthine or guanine to the corresponding nucleotides (so-called salvage pathway) but also the conversion of, e.g., the nontoxic purine analogue 6-thioguanine (6TG) to its ribophosphorylated derivative, which is then lethal to the cells. Therefore, a loss of the enzyme or its activity leads to resistance to 6TG, i.e. cells are unable to form lethal nucleotides and thus will grow and form colonies in the presence of the purine analogue. 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. The aim of the present study was to assess the potential of the test substance Isobutyraldehyde or its metabolite(s) to induce gene mutations in CHO cells. The study was initiated on February 22, 1999 and the practical work was completed on March 17, 1999 (1st experiment) and on May 18, 1999 (2nd experiment).