Perturbation of lysosomal degradation pathways by drugs such as chloroquine and vinblastine has given us important insights into the means for segregation and degradation of cytoplasm by the lysosomes. Vinblastine (VBL) is a microtubule depolymerizer which increases the number of autophagosomes and also impairs the maturation of the autophagosomes into secondary lysosomes. The authors have tested the effects of the microtubule stabilizer taxol on the proteolytic activity of the lysosomal system in VBL-treated mice. Liver proteins of CD-1 mice were radioisotopically labeled. Taxol and/or vinblastine were subsequently administered. Proteolysis in liver homogenates was determined during in vitro incubation by measuring the release of trichloroacetic acid-soluble radioactivity over time. Taxol enhanced proteolysis in homogenates from VBL-treated mice. The site of the enhanced proteolysis was lysosomal, because it was demonstrated in a crude lysosomal fraction, it manifested an acid pH optimum, and it could be suppressed with cycloheximide. Morphometric analysis indicated that taxol caused a shift in hepatocyte lysosomes of VBL-treated mice toward the later stages of the degradation cycle, coupled with a decrease in the number of small dense body profiles. The results indicate that taxol enhances the maturation of the autophagosomes into proteolytically active secondary lysosomes. It is concluded that the microtubule stabilizer taxol enhances fusion between VBL-induced autophagosomes and lysosomes (small dense bodies) and does not affect the segregation of cytoplasm into autophagosomes.