A metabolic activation system with rat-liver microsome fraction plus cofactors (S9 mix) was applied to chromosomal aberration tests in vitro for the screening of chemical mutagens or carcinogens in the environment. Dialkylnitrosamines only induced chromosomal aberrations in Chinese hamster cells (CHL) when treated with S9 mix. The incidence of chromosomal aberrations in CHL varied with experimental conditions, e.g. incubation time, recovery time, components of S9 mic and inducers used for preparation of S9. For dimethylnitrosamine (DMN), the maximal incidence was obtained when the cells were incubated with S9 mix for 3 h and harvested 24 h after treatment. Therefore, this system (3 h incubation and 24 h recovery) was routinely applied to further screening of other chemicals with S9 prepared from PCB-pretreated rats. 10 carcinogens (e.g. 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene, quinoline, etc.) out of 16 induced aberrations when they were treated with S9 mix, whereas the remaining 6 carcinogens (e.g., 3-methyl-cholanthrene, 4-o-tolylazo-o-toluidine, etc.) induced few or no aberrations even after activation. Two insecticides, allethrin and diazinon, were strongly positive at relatively low doses only when they were activated with the S9 mix. Medical drugs, such as ethenzamide, methyl p-hydroxybenzoate and nitrofurazone, and a food additive, sodium hypochlorite, were positive on activation. Chemicals used for industry, such as styrene monomer and tris-dichloropropylphosphate, were also positive in our activation system.