Ritonavir is an HIV protease inhibitor used in the therapy of HIV infection. Ritonavir has also been shown to inhibit the chymotrypsin-like activity of isolated 20S proteasomes. Here, we demonstrate that ritonavir, like classical proteasome inhibitors, has antitumoral activities. In vitro, ritonavir strongly reduced the rate of proliferation of several tumor cell lines and induced their apoptosis. Nontransformed cell lines and terminally differentiated bone-marrow macrophages were comparatively resistant to the apoptosis-inducing effect. In vivo, ritonavir, administered p.o. for a week at doses of 6-8.8 mg/mouse/day, caused significant growth inhibition (76-79% after 7 days of treatment) of established EL4-T cell thymomas growing s.c. in syngeneic C57BL/6 mice. Unexpectedly, we found that ritonavir activates the chymotrypsin-like activity of isolated 26S proteasomes, in strong contrast to its effect on isolated 20S proteasomes. The net effect of low micromolar concentrations of ritonavir on the chymotrypsin-like activity in cells and cell lysates was a weak inhibition, consistent with marginal alterations of polyubiquitinated proteins, marginal alterations in acid-soluble proteolytic peptide levels, and a small accumulation of the tumor suppressor protein p53, in cells treated with ritonavir. In contrast, we found a relatively strong accumulation of the cyclin-dependent kinase inhibitor p21(WAF-1), a sign of deregulation of cell-cycle progression typical for apoptosis induction in transformed cells by classical proteasome inhibitors. We demonstrate that p21 accumulation in the presence of ritonavir is attributable to the inhibition of proteolytic degradation. Accumulation of p21 most likely reflects a selective inhibition of proteasomes, in line with the atypical degradation of p21, which does not require ubiquitination. These findings suggest that selective perturbation of proteasomal protein degradation may play a role in the antitumoral activities of ritonavir.