Chromosome aberration and Ames tests were compared for screening carcinogens. Methods A fibroblast cell line, CHL, was used for chromosome aberration (CH) tests by a direct method or with activation by liver homogenate of Wistar rats pretreated with PCB. S. typhimurium, TA 98, TA 100, and TA 1537 were used for Ames test assays with or without metabolic activation systems. Results Carcinogens such as N-alkyl-N-nitrosoureas, N-alkyl-Nnitrosourethans and N-alkyl-N'-nitro-N-nitrosoguanidines were all positive both in the CH-test and in the Ames test after the direct exposure. Dialkylnitrosamines, benzo(a)pyrene; 7,12dimethylbenz(a)anthracene and styrene monomer were positive in both tests but only with metabolic activation. Urethane, barbital, diethylstilbestrol and maleic anhydride were negative in the Ames test but positive in the CH-tests. Sodium nitrite, sodium hypochlorite, cochineal, caramel and acid red were positive in both tests, while propyi gallate, sodium benzoate, sodium nitrate, potassium sorbate and saccharine sodium were positive only in the CH-tests. Caffeine, ethionamide, furosemide, ethenzamide, noscapine, urea, diazinon, saccharose, NaCl, Na nitrate, N ethyl-N'-nitroguanidine, methyl-p-hydroxybenzoate, hydrogen peroxide, 4-aminoguinoline-1-oxide and nitrosopropazine were positive only in the CH tests. Isoniazid, stevioside, nitrofuranloin, quinoline, sulpyrine, 4-nitroquinoline-l-oxide, tryptophan pyrolyeates, phenylenediamine, 2-amino-5-nitrophenol, nitrosocarbaryl, p-propiolactone, acetylaminofluorene, nitrophenylenediamine, rhodamine B. acrylonitrile, propane sultone, KBr and Tris-dichloropropylphosphate were positive in both tests. l Hydralazine-HCl, phenacetin were both positive, but acetaminophen, methylclothiazide, phenylbutazone, aspirin and LSD were only positive in the CH-tests. Captan, 2,2,2-trichloro-1-hydroxyethyl-dimethyl phosphate were positive in both tests, while Carbaryl, Maneb, m-tolyl-methyl carbamate and methyl propyltrimethylene carbamate were positive only in the CH-tests. Tris-dibromopropylphosphate and Pyrovatex CP, 2-(2-furyl)-3(5-nitro-2-furyl)-acrylamide and 2,2-dichlorovinyl dimethyl phosphate were also positive in both tests. There were approximately 10^6 fold differences between the lowest and the highest in the strength of clastogenicity in the CH test.