Cell cycle arrest in response to DNA damage involves protein stabilization and consequent upregulation of p53, which induces transcription of cyclin-dependent kinase inhibitor p21 (CDKN1A). We now show that p21 acts as a negative regulator of the cellular levels of p53. p21 knockdown by short hairpin RNA strongly increased p53 upregulation by a DNA-damaging drug doxorubicin in HT1080 fibrosarcoma cells. A protease inhibitor N-Ac-Leu-Leu-norleucinal (ALLN) drastically increased the amount of p53 in HCT116 colon carcinoma cells, but it had no effect on the already high p53 level in a p21(-/-) derivative of this cell line. Inhibition of transcription, which increases p53 levels in different cell lines due to the degradation of p53-destabilizing proteins such as Mdm2, failed to increase but instead decreased the amount of p53 in p21(-/-) cells, despite a drastic decrease in the level of Mdm2. These results indicate that p21 acts as a negative regulator of p53 stability in different cell types. p53 regulation by p21 may provide a negative regulatory loop that limits p53 induction.