We have investigated the interactions occurring between the prelysosomal compartment, PLC/late endosome, and terminal lysosomes using an approach that allowed us to internalize and deliver specific tracers to these compartments, and look for evidence of their meeting. After internalization of sucrose, the lysosomes (sucrosomes), but not the PLC/late endosomes, became significantly swollen and visible in the light microscope. If invertase is then added to the medium it reaches the lysosomes where it cleaves sucrose into fructose and glucose. These sugars, unlike sucrose, can be transported into the cytosol, resulting in the disappearance of the sucrosomes. We previously showed that phagocytosed latex beads are delivered specifically to, and reside in, the PLC/late endosome, a stage earlier than the lysosomes in the endocytic pathway (Rabinowitz et al. (1992) J. Cell Biol. 116, 95-112). In the present study, we demonstrate that invertase conjugated to the latex beads, and thus immobilized in late endosomes, has access to the sucrose present in the more distal lysosomes. Experiments using nocodazole indicate that this retrograde fusion event requires the presence of an intact microtubule network. The simplest interpretation of our results is that the two compartments fuse, allowing for a retrograde transport of sucrose from the lysosomes to the PLC/late endosomes.