The RNA tumor viruses, Kirsten murine sarcoma virus (KiMSV) and Abelson murine leukemia virus, are shown to produce transforming factors (TFs) similar to sarcoma growth factor (SGF) produced by Moloney murine sarcoma virus-transformed 3T3 cells. TF production by KiMSV-transformed cells is dependent on the presence of a functional viral genome. TFs from KiMSV-transformed cells induce the transformed phenotype in normal cells, which suggests that the TFs play a integral role in transformation. We have extended TF production to DNA tumor viruses by demonstrating that cells transformed by simian virus 40 (SV40) and polyoma virus produce a growth factor or factors that render Rat-1 cells anchorage independent. The SV40-induced TF is not related to the KiMSV-induced TF. Last we demonstrate that cells vary in their sensitivity to TFs and propose that tumor cells have an increased responsiveness to TF-induced anchorage-independent growth.