Exosomes are small membrane vesicles (50-90 nm in diameter) secreted by most hematopoietic cells. We provide here the first evidence for the presence of exosomes in vivo, in the blood. Plasma samples of all healthy donors tested (n = 15) contain vesicles that are similar in shape, size and density to the previously described exosomes. They were clearly identified by electron microscopy after isolation by differential ultracentrifugation or immunoisolation with CD63-coated latex beads. We performed their biochemical characterization by western blot analysis and by flow cytometry after vesicle adsorption onto latex beads using a panel of mAbs. We observed that these plasma-derived vesicles contain tetraspanin molecules such as CD63, CD9, CD81 as well as class I and class II MHC molecules and Lamp-2 (i.e. proteins that are known to be enriched in exosomes). In addition, these vesicles float on sucrose gradient at a density similar to exosomes. Our results demonstrate that blood is a physiological fluid for exosome circulation in the body, suggesting their role in cell-cell or organ-organ communications as carriers for molecules that need to reach distant cell targets.