CRL7800 cells are of human testicular cancer origin and are sensitive to killing by ionizing radiation. After transfection with a plasmid expressing the T-antigen (pSC), cells show enhanced growth and an increased resistance to ionizing radiation. Cell cycle analysis reveals perturbation of a cell cycle checkpoint which, after irradiation, results in an increase in G2-phase arrest in CRL7800VA cells. These experiments demonstrate the modulation of radiation sensitivity and cell cycle arrest of human tumor cells by the introduction of viral genes.