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Proteasomes, the proteolytic machinery of the ubiquitin/ATP-dependent pathway, have a relevant role in many processes crucial for cell physiology and cell cycle progression. Proteasome inhibitors are used to block cell cycle progression and to induce apoptosis in certain cell lines. Here we examine whether proteasomal function is affected by the anti-tumour drug vinblastine, whose cytostatic action relies mainly on the disruption of mitotic spindle dynamics. The effects of vinblastine on the peptidase activities of human 20S and 26S proteasomes and on the proteolytic activity of 26S proteasome were assessed in the presence of specific fluorogenic peptides and 125I-lysozyme–ubiquitin conjugates respectively. The assays of ubiquitin–protein conjugates and of inhibitory κBα (IκBα), which are characteristic intracellular proteasome substrates, by Western blotting on lysates from HL60 cells incubated with or without vinblastine, illustrated the effects of vinblastine on proteasomes in vivo. We also evaluated the effects of vinblastine on the signal-induced degradation of IκBα. Vinblastine at 3–110μM reversibly inhibited the chymotrypsin-like activity of the 20 S proteasome and the trypsin-like and peptidyl-glutamyl-peptide hydrolysing activities of both proteasomes, but only at 110μM vinblastine was the chymotrypsin-like activity of the 26S proteasome inhibited; furthermore, at 25–200μM the drug inhibited the degradation of ubiquitinated lysozyme. In HL60 cells exposed for 6h to 0.5–10μM vinblastine, the drug-dose-related accumulation of polyubiquitinated proteins, as well as that of a high-molecular-mass form of IκBα, occurred. Moreover, vinblastine impaired the signal-induced degradation of IκBα. Cell viability throughout the test was approx. 95%. Proteasomes can be considered to be a new and additional vinblastine target.