Ornithine decarboxylase (ODC) is the most notable example of a protein degraded by the 26 S proteasome without ubiquitination. Instead, ODC is targeted to degradation by direct binding to a polyamine-induced protein termed antizyme (Az). Antizyme inhibitor (AzI) is an ODC-related protein that does not retain enzymatic activity yet binds Az with higher affinity than ODC. We show here that like ODC, AzI is also a short-lived protein that undergoes proteasomal degradation. However, in contrast to ODC degradation, the degradation of AzI is ubiquitin-dependent and does not require interaction with Az. Moreover, Az binding actually stabilizes AzI by inhibiting its ubiquitination. Substituting the C terminus of AzI with that of ODC, which together with Az constitutes the complete degradation signal of ODC, does not subvert AzI degradation from the ubiquitin-dependent mode to the Az-dependent mode, suggesting dominance of the ubiquitination signal. Our results suggest opposing roles of Az in regulating the degradation of AzI and ODC.