Various experiments have demonstrated a collaborative action of Myc and Ras, both in normal cell growth control as well as during oncogenesis. We now show that Ras enhances the accumulation of Myc activity by stabilizing the Myc protein. Whereas Myc has a very short half-life when produced in the absence of mitogenic signals, due to degradation by the 26S proteasome, the half-life of Myc increases markedly in growth-stimulated cells. This stabilization is dependent on the Ras/Raf/MAPK pathway and is not augmented by proteasome inhibition, suggesting that Ras inhibits the proteasome-dependent degradation of Myc. We propose that one aspect of Myc-Ras collaboration is an ability of Ras to enhance the accumulation of transcriptionally active Myc protein.