Intraneuronal accumulation of ubiquitin conjugates in inclusion bodies and neurofibrillary tangles is a pathological feature of neurodegenerative disorders such as Alzheimer's disease and Down's syndrome and of normal aging of the brain. Amyloid beta-protein (A beta) and its precursor are found in neurofibrillary tangle-containing neurons. A beta is the major component of extracellular plaques. We showed that A beta acts as an inhibitor of the ubiquitin-dependent protein degradation in vitro. We examined the effect of A beta on the steps of this proteolytic pathway that contribute to the level of ubiquitin conjugates in the cell. Neither conjugate formation nor conjugate deubiquitination was affected by the presence of A beta. However, A beta significantly reduced the rate of conjugate degradation. Our results indicate that A beta interacts with the proteolytic step of the ubiquitin degradative pathway. Since this step is performed by the 26 S proteasome, the effect of A beta on the catalytic core of this proteolytic complex, the 20 S proteasome, was determined. We found that A beta selectively inhibits the chymotrypsin-like activity of the 20 S proteasome. Under pathological conditions in the affected neuron, A beta could interfere with ubiquitin-dependent degradation by inhibiting the 26 S proteasome activity. This finding may explain the origin of the accumulation of ubiquitin conjugates.