We have addressed the following question: what is the mechanism behind the delivery of internalized molecules from mature endosomes to lysosomes in HEp-2 cells, and which role does the cytoskeleton play in this process? Quantitative electron microscopy and immunogold labeling revealed that whereas the cytoskeleton was not of importance for endosome maturation, actin filaments facilitated fusion of mature endosomes with preexisting lysosomes. Delivery to lysosomal degradation was not dependent on protein synthesis as determined biochemically, but was reduced by cytochalasin D. Observations made by electron microscopy as well as by video microscopy of living cells showed that the concerted action of actin filaments and microtubules was responsible for the random distribution and movement of endocytic organelles throughout the cell. Actin microfilaments, however, seem to facilitate perinuclear clustering and frequent fusion of mature endosomes and lysosomes, while microtubules play a role in preventing formation of large lysosome aggregates by separating endosomes and lysosomes and moving them toward the cell periphery. Taken together, our data suggest that delivery of internalized molecules to lysosomal proteolysis takes place by fusion of mature endosomes with preexisting lysosomes and that actin microfilaments somehow facilitate this step.