The alkaline single cell gel electrophoresis (SCG test or comet assay) was used to characterize the influence of sodium arsenite (NaAsO?) and cadmium sulphate (CdS04) on the persistence of mutagen-induced DNA lesions. Human blood and SV40transformed fibroblasts (MRCSCVI ) were treated for 2 hr with methyl methanesulphonate (MMS) or benzo(a)pyrene (BaP). MMS induced concentration-related DNA damage in white blood cells (WBC) and fibroblasts in similar concentrations. For the induction of DNA damage by BOP, higher concentrations had to be applied to WBC than to the fibroblast cell line. To study the influence of metal ions on the persistence of DNA lesions, treated cells were further incubated for 2 hr in the absence (postincubation) or presence (posttreatment) of NaAs02 or CdSO4. After postincubation, MMS and BaPinduced DNA effects were reduced in both cell types, indicating that repair of DNA lesions had taken place. When the cells were posttreated with NaAs02 or CdS04, BaP-and MMSinduced DNA lesions persisted in both cell types, indicating an inhibition of DNA repair by these metals. The results suggest a strong interaction of arsenic and cadmium with BaP-and MMS-induced DNA repair prccesses.