The effect of x-irradiation dose rate on the clonagenic survival of mouse embryo fibroblast cell line NIH/3T3 and its N-ras human oncogene transformed subline was studied. Both control and N-ras transformed cell lines were maintained in Dulbecco's modified Eagle's medium at 37 degrees C with 5% CO2. These cell lines were passaged twice weekly and the cells were irradiated in log phase on a 250 kVp orthovoltage unit at 5 or 200 rad/min, adjusting filtration and FSD to account for each dose rate. After irradiation, the cells were replated and colonies of greater than or equal to 50 cells were scored on day 7. D0 and n were calculated via linear regression analysis. There was a significant increase in saturation density and plating efficiency of N-ras transformed cells with loss of contact inhibition. There was no significant difference in radiosensitivity between the two cell lines at 5 rad/min. For NIH/3T3 D0 = 336, n = 2.19; for N-ras transformant D0 = 314, n = 2.35 (p = 0.65); however, irradiation at 200 rad/min revealed a significant survival advantage for the transformed line. For NIH/3T3 D0 = 145, n = 9.1 and for the N-ras transformed line D0 = 208, n = 4.05 (p = 0.0018). The data provide evidence that repair factors which govern irradiation survival may differ for high and low dose rate irradiation and that repair of high dose rate irradiation damage is enhanced directly or indirectly by expression of the N-ras oncogene. The data support hyperfractionated (low dose rate) irradiation for improving the therapeutic ratio during control of rapidly proliferating tumors expressing an activated N-ras oncogene.