Protein degradation occurs more rapidly in senescent WI-38 cultures than in phase II cultures or in SV-40 transformed WI-38 cells (VA-13). The first differences are found in early phase III, when short lived but not long lived proteins are degraded more rapidly. At the end of phase III long lived proteins are also degraded more rapidly as shown by both intermittent perfusion and approach to equilibrium methods. By both methods the rates of protein degradation for the virally transformed derivative are the same as those for phase II WI-38, implying that transformation has not altered these characteristics of protein degradation. WI-38 cells incorporate canavanine, an analog of arginine, into protein. This analog, as well as p-fluorophenylalanine and azetidine carboxylic acid, accelerates the degradation of proteins labeled with [3H]leucine in their presence but does not alter the degradation rates of proteins prelabeled with [14C]leucine in the absence of the analogs. These results imply that the analogs increase the intracellular degradation rates of proteins because they render them more susceptible to the degradative system. Late phase III WI-38 cells may not selectively catabolize proteins containing canavanine as rapidly as do phase II and VA-13 cells. These results imply that the phase III protein degradative system becomes partially defective, thereby losing its ability to rapidly catabolize altered protein which leads to increased levels of abnormal proteins and decreased cell function.