From extracts of a hamster cell line transformed by the Simian virus 40, we have shown the presence of a factor that stimulates the replication of K virus and also counteracts the mechanism of action of interferon. The sensitivity of this factor to a proteolytic enzyme is a possible indication that we are dealing with a protein. This "stimulon" is unrelated to the T antigen as shown by chromatography on Sephadex G-100 and by complement fixation test. Furthermore, at 56 °C, all direct effects on virus K replication are lost but the anti-interferon activity is preserved.