Newly synthesized apolipoprotein B (apoB) is degraded by a proteolytic process in the pre-Golgi compartment that can be inhibited by N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal (ALLN) but not by several other protease inhibitors. We have tested the hypothesis that the ubiquitin-proteasome pathway is involved in the intracellular degradation of apoB in liver cells. We found that inhibitors of proteasomes blocked the degradation of apoB in cultured human hepatoma (HepG2) cells. Protein degradation by proteasomes is ATP-dependent, and ATP depletion by dinitrophenol and 2-deoxyglucose also inhibited apoB degradation in these cells. Furthermore, the intracellular human apoB isolated by immunoprecipitation was shown to react specifically with anti-ubiquitin antibody by immunoblotting. This result was corroborated by sequential immunoprecipitation of [35S]methionine-labeled proteins by anti-human apoB and anti-ubiquitin antisera. In contrast, secreted apoB was not ubiquitinated. The amount of intracellular ubiquitinated apoB was increased by the proteasome inhibitors, ALLN and carbobenzoxyl-leucinyl-leucinyl-norvalinal-H (MG115). Our findings suggest that the ubiquitin-proteasome pathway is one mechanism for the intracellular degradation of apoB.