Three normal human diploid cell strains were transfected with an activated Ha-ras oncogene (EJ ras) or SV40 T-antigen. Multiple clones were examined for morphological alterations, growth requirements, ability to grow under anchorage independent conditions, immortality and tumorigenicity in nude mice. Clones expressing SV40 T-antigen alone or in combination with ras protein p21 were significantly radioresistant as compared with their parent cells or clones transfected with the neo gene only. This radioresistant phenotype persisted in post-crisis, immortalized cell lines. Cells transfected with EJ ras alone showed no morphological alterations nor significant changes in radiosensitivity. Cell clones expressing ras and/or SV40 T-antigen showed a reduced requirement for serum supplements, an increase in aneuploidy and chromosomal aberrations, and enhanced growth in soft agar as an early cellular response to SV40 T-antigen expression. The sequential order of transfection with SV40 T-antigen and ras influenced radio-sensitivity but not the induction of morphological changes. These data suggest that expression of the SV40 T-antigen but not activated Ha-ras plays an important role in the radiosensitivity of human diploid cells. The radioresistant phenotype in SV40 T transfected cells was not related to the enhanced level of genetic instability seen in pre-crisis and newly immortalized cells, nor to the process of immortalization itself.