Amphipols are a new class of surfactants that make it possible to handle membrane proteins in detergent-free aqueous solution as though they were soluble proteins. The strongly hydrophilic backbone of these polymers is grafted with hydrophobic chains, making them amphiphilic. Amphipols are able to stabilize in aqueous solution under their native state four well-characterized integral membrane proteins: ( i ) bacteriorhodopsin, ( ii ) a bacterial photosynthetic reaction center, ( iii ) cytochrome b ₆ f , and ( iv ) matrix porin.