Proteasomes constitute the degradative machinery of the ubiquitin/adenosine triphosphate-dependent proteolytic pathway, which is involved in many cell functions, including immune response and apoptosis, and in HIV maturation and infectivity. To examine whether proteasomes are targeted by antiretroviral agents. Chymotrypsin-like, trypsin-like and peptidyl-glutamyl-peptide hydrolysing activities of purified human 26S and 20S proteasomes, the latter depleted or enriched in 11S regulator, were assayed after incubation with indinavir, lamivudine and zidovudine at 1-80 microM alone and in combination. To assess the drug effects on cellular functions regulated by proteasomes, the accumulation of ubiquitin-tagged proteins, the processing of the nuclear factor kappa B precursor p105, and the degradation of the inhibitor of nuclear factor kappa B, isoform alpha (IkappaBalpha) were evaluated by Western immunoblotting in Jurkat cells after incubation for 6 h with the drugs above. Trypsin-like and mostly chymotrypsin-like activities of purified 26S proteasome were inhibited by each drug from 10 to 80 microM, more by double combinations and mostly by the triple combination. The peptidyl-glutamyl-peptide hydrolysing activity of the 26S proteasome and the three peptidase activities of the 20S proteasome, depleted or enriched in 11S regulator, were unaffected. The accumulation of ubiquitin-tagged proteins, reduced IkappaBalpha degradation and p105 processing were appreciable in intact cells with the triple drug combination. The human 26S proteasome is a target of antiretroviral agents. This suggests that the antiviral action and some clinical and immunological benefits of combined antiretroviral therapy rely not only on its known effects on viral enzymes, but also on host cell components.