The mutagenic activity of pBR322 bacterial plasmid DNA and of pBR322ins and pBR322insN recombinant plasmid DNAs has been investigated in Blld-ii-FAF 28 line cultivated fibroblasts of Chinese hamster. The pBR322 bacterial plasmid DNA is shown to induce no resistant mutations to 6-mercaptopurine; the pBR322insN (the frameshift mutation in human insulin gene; this gene is supposed to have no expression in the cells because of frameshift mutation) induces neither such mutations as well. The pBR322ins recombinant plasmid carrying the native human insulin gene induces the gene mutations in this system. The influence of the insulin gene structure on the gene mutations induction is taken into account. The results obtained and the data of other authors illustrate a great interest of recombinant DNA molecules carrying transgenes in the field of mutation research.