We have investigated the effects of viral Kirsten ras oncogene expression in Madin-Darby canine kidney (MDCK) II epithelial cell on the differential protein expression of organelle proteins. MDCK cells, stably transformed via infection with a helper-independent retroviral vector construct, were grown on permeable filter supports. Whereas normal cells form highly polarized monolayers, ras-transformed cells display an unpolarized phenotype, detaching from the substratum and developing multilayers (Schoenenberger, C.-A. et al., J. Cell Biol. 1991, 112, 873–889). We postulate that this breakdown of epithelial polarity reflects disturbed intracellular protein transport and sorting, namely, proteins will no longer be sorted correctly in intracellular organelles and will therefore not reach their appropriate target membrane. Here we emphasize the role of endosomes as sorting platform in epithelial cells. We found significant differences in the molecular composition of endosomes from normal vs. oncogenic transformed epithelial cells, strengthening previous evidence indicating that oncogenic transformation results in abnormal expression of normal genes (Celis, J. E., Olsen, E., Electrophoresis 1994, 15, 309–344) as well as the expression of new ones (Huber, L. A. et al., Electrophoresis 1994, 15, 468–473).