The repair of UV-irradiated DNA of plasmid pBB29 was studied in an incision-defective rad3-2 strain of Saccharomyces cerevisiae and in a uvrA6 strain of Escherichia coli by the measurement of cell transformation. Plasmid pBB29 used in these experiments contained as markers the DNA of nuclear yeast gene LEU-2 and DNA of the bacterial plasmid pBR327 with resistance to Tet and Amp enabling simultaneous screening of transformant cells in both microorganisms. We found that the yeast rad3-2 mutant, deficient in incision of UV-induced pyrimidine dimers in nuclear DNA, was fully capable of repairing such lesions in plasmid DNA. The repair efficiency was comparable to that of the wild-type cells. The E. coli uvrA6 mutant, deficient in a specific nuclease for pyrimidine dimer excision from chromosomal DNA, was unable to repair UV-damaged plasmid DNA. The difference in repair capacity between the uvrA6 mutant strain and the wild-type strain was of several thousand-fold. It seems that the rad3 mutation, which confers deficiency in the DNA excision-repair system in yeast, is limited only to the nuclear DNA.