The single cell gel test (SCG-test or comet assay) is a rapid and sensitive 
method for measuring DNA damage and repair in individual cells. A wide variety 
of mutagens have been shown to cause DNA alterations detectable with the comet 
assay, but it is not yet clear whether a relationship exists between the DNA 
effects and the induction of mutations. We are therefore investigating in a cell 
culture system with human cells (MRC5CV1) the induction of DNA damage by 
environmental mutagens and the formation of mutations at the HPRT gene. In the 
present study we investigated benzo[a]pyrene (BP), an environmental mutagenic 
and carcinogenic polycyclic aromatic hydrocarbon, and its reactive metabolite 
(+)-anti-benzo[a]pyrene-7,8-diol 9, 10-oxide ((+)-anti-BPDE). S9 mix activated 
BP and the direct acting mutagen (+)-anti-BPDE caused a concentration-related 
increase in DNA migration in the comet assay. Postincubation experiments 
indicated that induced DNA effects are eliminated by DNA repair within 24 h. 
BP-treatment caused a strong genotoxic effect in the comet assay but had only a 
marginal effect on the frequency of gene mutations. When cells were treated with 
BP in the presence of cadmium sulphate, a clear increase in genotoxicity was 
observed while the effect on mutations was unchanged. Our results indicate that 
DNA alterations detected with the comet assay do not necessarily relate to 
mutagenesis. The absence of a close relationship between DNA migration in the 
comet assay and mutagenesis may be explained by the fact that some effects seen 
in the comet assay occur as a consequence of an error free DNA repair process.