In mammals, polysialic acid (polySia) attached to a small number of transmembrane protein carriers occurs on the surface of plasma membranes of neural, cancer, immune, and placental trophoblast cells. Here, our goal was to demonstrate the presence of polySia on exosomes and its effect on membrane properties. We isolated exosomes and found that polysialylated exosomes in fetal bovine serum originate mostly from placental trophoblasts, while in calf bovine serum, they originate from immune cells. Enzymatic removal of polySia chains from the exosomal surface makes the membrane surface potential more positive, transmembrane potential more negative, and reduces the activation energy for membrane anisotropy changes. We demonstrate for the first time that exosomes could interact through polySia-raft interactions. We suggest that polysialylation of exosomal membrane can have a thermo-protecting effect and can modulate exosome-plasma membrane interactions.Exosomes are small extracellular vesicles (EVs), 50 Ä 150 nm in diameter, that originate from the endosome and are released from cells when multivesicular bodies (MVBs) containing intraluminal vesicles (ILVs) fuse with the plasma membrane. Microvesicles (100 Ä 1000 nm in diameter) are larger than exosomes and are released from cells through blebbing (budding out) and fission of the plasma membrane. There is a small overlap of exosomes and microvesicles concerning their size; however, vesicles within the diameter range of 50 Ä 100 nm are considered as exosomes. In addition to exosomes and microvesicles, there are also other nonvesicular nanoparticles in the body fluids, for example, lipoproteins and their complexes [1]. Exosomes are mainly recovered after high-speed ultracentrifugation, but such preparations may include the smallest non-EV structures, for example, high-density lipoprotein (5 Ä 35 nm in diameter) [1]. However, lipoproteins can be separated from exosomes by gel filtration method using appropriate matrix [2,3]. Exosomes are involved in mammalian pathophysiology including cancer, immune responses, cardiovascular diseases, regeneration, and stem cell-based therapy (reviewed in ref. [4]). Exosomes in cancer seem to be Abbreviations ANS, 8-anilino-1-naphthalenesulfonic acid ammonium salt