Rat embryo cells or Rat-1 fibroblasts were transfected with either an activated c-myc or a c-Ha-ras from the T24/EJ bladder carcinoma, or they were cotransfected with both. A gene conferring neomycin or hygromycin resistance was also cotransfected so that independent cell lines could be selected by growth in medium containing the antibiotic. Certain isolates from cells transfected with only one type of oncogene were further transformed by exposure to 600 cGy of 250-kVp X-rays. Successful transfection and transformation were characterized by altered morphology, increased plating efficiency, shorter doubling time, longer life span, foci formation, anchorage-independent growth, and Southern and Northern hybridization analysis. The thermal response of these cells at different stages of oncogenic transformation was examined by exposing exponentially growing cells to 45 degrees C for 0 to 45 min and measuring cellular survivals using colony formation assay. We found that cells transfected with myc oncogene, singly or in combination with ras, were more sensitive to thermal stress. Aside from that, the cells' thermal sensitivity was not affected by the degree or the nature of transformation.