Dot blot hybridization was used to determine the transcript levels of 10 cellular oncogenes in Fischer rat fibroblasts transformed or infected in tissue culture by polyoma virus or SV40. The level of these messages was compared to that determined for nontransformed, noninfected control cells. The analysis includes the oncogenes myc, sis, mos, erbB, erbA, Ha-ras, Ki-ras, src, fps, and abl. For all the oncogenes tested, except for mos, detectable expression was observed in all cell lines studied including the normal control cells; when normal and transformed or infected cells were compared, no significant difference in transcript level was found for any of the oncogenes except one. A slight elevation of sis message was observed for some transformants. The results of this study apply to six polyoma and seven SV40 transformants which were chosen with the purpose of analyzing transformants displaying a variety of properties. Thus, the polyoma-transformed cell lines varied in their expression of the transformed phenotype as judged by anchorage-independent growth and cell morphology, in their coding capacity for and expression of early gene products, and included two classes of rat fibroblasts transformed by ts-a mutants: those with a temperature-insensitive transformed phenotype, and those with a temperature-sensitive phenotype, A-type and N-type, respectively. Concerning the latter two types, no differences in oncogene expression were observed between cells grown at low and those grown at high temperatures, or between the two groups of cells at either temperature.