Chromosomal aberration tests in vitro were carried out on Chinese hamster cells grown in culture with various chemicals, including carcinogenic N-nitroso compounds and their related derivatives, food additives, medical drugs, pesticides and other chemicals commonly used in laboratories or industries. Sixty-three of the 134 chemicals gave negative results in our test system even with doses at which the cell growth was markedly inhibited. Nearly all compounds known to be mutagenic in bacteria were also positive in our tests. Both urethane and diethylstilbestrol were positive, even though they are known to be carcinogenic but not mutagenic in bacteria. Compounds such as N-alkyl-N'-nitroguanidines, barbital, sodium benzoate, saccharin sodium, sodium nitrite, sodium nitrate and 4-aminoquinoline-1-oxide were positive in our chromosome tests, but they have not been conclusively tested for their carcinogenicity.