We have examined one of the most striking characteristics of the c-myc oncogene protein product: its extremely short half-life relative to all but a few normal cellular proteins. Our studies indicate that the rapid degradation of c-myc proteins is not confined to human and avian cells, where it was first observed, but is found in cells derived from as evolutionarily widely separated species as murine and amphibian as well as in both normal and transformed cells. In addition the rate of degradation appears to be essentially the same throughout the G1, S, and G2 phases of the cell cycle as determined by "pulse-chase" analysis of cell cycle subpopulations separated by centrifugal elutriation. Kinetic analysis of c-myc protein turnover utilizing immunoprecipitation from labelled cells or immunoblotting of total nuclei indicate that there is not likely to be a significantly large pool of the protein which is protected from degradation and that no major degradation products can be detected. Evidence that the nucleus plays a role in degradation comes from preliminary experiments which indicate that in enucleated cells the c-myc proteins are stable. We also discuss evidence suggesting that myc proteins are stable in mitotic cells. Finally, using a series of chemical agents we have found conditions which inhibit in vivo degradation. These experiments indicate the involvement of both ATP and metal ions in degradation of c-myc proteins.