Multivesicular bodies (MVB) are endosomal compartments that contain multiple vesicles, which derive from a delimiting membrane by inward budding. Incorporation of membrane proteins into the luminal vesicles requires, at least for some model proteins, monoubiquitination of their cytoplasmic domain. The ubiquitin tags are recognized by a sorting machinery, of which some components are also monoubiquitinated. The ubiquitin tags and the sorting machinery are both removed before the vesicles bud into the MVB lumen. MVB vesicles are therefore not expected to contain monoubiquitinated proteins. The MVB content is degraded upon fusion of MVB with lysosomes. In many cell types, however, MVB can also fuse with the plasma membrane, resulting in secretion of their luminal vesicles into the extracellular milieu. Such secreted vesicles are termed exosomes, and their protein composition should, due to their origin, be identical to that of MVB luminal vesicles. We here demonstrate that exosomes contain polyubiquitinated proteins, many of which are not integrated into the membrane and relatively enriched as compared to total cell lysates. These results suggest that a subset of polyubiquitinated cytoplasmic proteins is incorporated into the MVB pathway. The potential cell biological relevance of this observation is discussed. Furthermore, these data indicate that ubiquitinated proteins can serve as markers for exosomes.