The interaction between late endocytic structures and microtubules in polarized cells was studied using a procedure previously shown to cause microtubule-dependent redistribution of lysosomes in fibroblasts and macrophages (Heuser, J. 1989. J. Cell Biol. 108:855-864). In cultured rat hippocampal neurons, low cytoplasmic pH caused cation-independent mannose-6-phosphate receptor-enriched structures to move out of the cell body and into the processes. In filter grown MDCK cells lowering the cytosolic pH to approximately 6.5 caused late endosomes to move to the base of the cell and this process was shown to be microtubule dependent. Alkalinization caused a shift in distribution towards the apical pole of the cell. The results are consistent with low pH causing the redistribution of late endosomes towards the plus ends of the microtubules. In MDCK cells the microtubules orientated vertically in the cell may play a role in this process. The shape changes that accompanied the redistribution of the late endosomes in MDCK cells were examined by electron microscopy. On low pH treatment fragmentation of the late endosomes was observed whereas after microtubule depolymerization individual late endosomal structures appeared to fuse together. The late endosomes of the MDCK cell appear to be highly pleomorphic and dependent on microtubules for their form and distribution in the cell.