ALpha-synuclein (alpha-syn) has been known to be a key player of the pathogenesis of Parkinson's disease and has recently been detected in extracellular biological fluids and shown to be rapidly secreted from cells. The penetration of alpha-syn into cells has also been observed. In this study, we observed that dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, a glucosyltransferase inhibitor, and proteinase K inhibited the internalization of extracellular monomeric alpha-syn into BV-2 cells, and the addition of monosialoganglioside GM1 ameliorated the inhibition of alpha-syn internalization in dl-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol-treated BV-2 cells. Furthermore, inhibition of clathrin-, caveolae-, and dynamin-dependent endocytosis did not prevent the internalization of alpha-syn, but disruption of lipid raft inhibited it. Inhibition of macropinocytosis and disruption of actin and microtubule structures also did not inhibit the internalization of alpha-syn. In addition, we further confirmed these observations by co-culture system of BV-2 cells and alpha-syn-over-expressing SH-SY5Y cells. These findings suggest that extracellular alpha-syn is internalized into microglia via GM1 as well as hitherto-unknown protein receptors in clathrin-, caveolae-, and dynamin-independent, but lipid raft-dependent manner. Elucidation of the mechanism involved in internalization of alpha-syn should be greatly helpful in the development of new treatments of alpha-syn-related neurodegenerative diseases.