The cellular Ha-ras oncogene, activated by missense mutations, has been implicated in intrinsic resistance to ionizing radiation. This study shows that the overexpression of the unmutated gene (proto-oncogene) may also be involved in how the cells respond to radiation. The experimental system consisted of mouse NIH 3T3-derived cell lines which carry multiple copies of a transcriptionally activated human c-Ha-ras proto-oncogene. Both tumorigenic (RS485) and revertant nontumorigenic subclones (PR4 and 4C3) which have high levels of ras expression exhibited a marked increase in radioresistance as measured by D0 compared to control NIH 3T3 cells. Other nontransformed cells with elevated levels of ras (phenotypically revertant line 4C8-A10) also had a significantly increased resistance to radiation, further indicating an association between ras and radioresistance. The increased radioresistance of the RS485 and phenotypic revertants could not be explained by a differential expression of the myc or metallothionein I genes or by variations in cell cycle. The correlation between increased ras proto-oncogene expression and radioresistance suggests that the ras encoded p21, a plasma membrane protein, may participate in the cellular responses to ionizing radiation.