We evaluated the effect of H-ras oncogene expression on resistance to ionizing radiation in cultured rat fibroblasts. The Rat-1 cell line, and two Rat-1 derivatives, MR4 and MR7, carrying a ZN-regulatable metallothionein-rasT24 fusion gene were used to study the effects of the ras oncogene on radiation sensitivity. Cells were irradiated with a 137Cs source (450 cGY/min) in the presence or absence of ZnSO4. Multiple cell survival studies did not show an appreciable difference in sensitivity to radiation among the lines in the presence or absence of ras oncogene expression.