Exosome are cell-derived vesicles formed by a lipid bilayer membrane, and its diameter is 40-100 nm. Exosomes are observed in body fluid, such as saliva, blood, urine, amniotic fluid, malignant ascites, and are secreted from cultured cells. Recently, it has been shown that exosomes include various proteins and RNAs, which have a possibility to function in intercellular signal transduction. These products are the antibodies which can specifically detect CD9, CD63 and CD81 known as exosome markers. These antibodies are suitable for isolation of exosome by immunoprecipitation.