Exponentially growing Balb/3T3 mouse fibroblasts contain protein populations with slow and fast turnover. These two stability classes were labelled selectively with 3H-leucine. The intracellular degradation of the proteins was then followed as the release into the medium of radioactive leucine. The degradation rate of both stability classes of protein is increased by about 55% in cultures whose growth is inhibited by high cell density. Serum-deprivation, which also halts cell growth, accelerates protein breakdown to a smaller extent, the increases for relatively stable and unstable proteins being 30% and 13%, respectively. The density-dependent increase in protein breakdown is also found in BHK21 cells but not in chick fibroblasts. Protein degradation in Balb/3T3 cells transformed by simian virus 40 is affected by serum-deprivation but not by cell density. The proteins which are relatively stable during growth were shown to become less stable in density-inhibited or serum-deprived cultures, and vice versa. Cycloheximide inhibits degradation to a variable extent. Dibutyryl adenosine-3',5'-cyclic monophosphate has no effect on the protein degradation under the conditions investigated here.