Ionizing radiation induces a number of molecular changes in cells, including DNA damage, mutation, genetic recombination, gene amplification, and chromosomal rearrangement. The studies described here make use of the process of DNA-mediated gene transfer to examine the molecular effects of ionizing radiation. Two Chinese hamster ovary cell lines, the wild-type, AA8-4, and a DNA repair-deficient line, EM9-1, that is sensitive to ionizing radiation, were transfected with the recombinant DNA plasmid, pSV2-GPT, either in the absence or presence of high-molecular-weight carrier DNA. Following transfection, cell populations were irradiated with graded doses of 137Cs gamma-rays. Results demonstrate that, on a per viable cell basis, ionizing radiation hinders the transfection of this plasmid when tested in the presence of carrier DNA. A similar dose response was seen for both the wild-type (AA8-4) and mutant (EM9-1) lines. However, in the absence of carrier DNA, 137Cs gamma-rays clearly enhanced the gene transfer process. An enhancement factor of 3-5 was seen for AA8-4 cells and 2-3 for EM9-1 cells. This enhancement occurred at relatively low doses (e.g., 50 cGy) and was not substantially elevated by larger doses.