Here are reviewed the most interesting results which have been obtained with a mutational assay based on the use of Simian Virus 40 (SV40) as a biological probe. This mutational assay allowed us first to study the mutation potency of some chemical and physical DNA damaging agents such as acetoxy-acetylaminofluorene and UV-light and of apurinic sites created by heat treatment under acidic conditions, and second to study at the molecular level the modifications induced by these treatments. A correlation between the location of the DNA adducts and the location of the hot spots of mutagenesis has tentatively been researched. No direct link has been found. Our results suggest that mutation hot spots are correlated with local DNA conformations which could be modified by the DNA damaging agents.