One-third of eukaryotic proteins are integrated within membranes, as are the targets of 40% of approved drugs. However, the lack of a general means of solubilizing, stabilizing and structurally characterizing active membrane proteins has frustrated efforts to understand their mechanisms and exploit their potential value. Here we report that bilayer disks formed by phospholipids and styrene maleic anhydride copolymer preserve the functional and structural integrity of alpha-helical and beta-barrel transmembrane proteins. They form 11 nm particles that are monodispersed, biocompatible, thermostable, and water-soluble, allowing diverse membrane proteins to be simply and rapidly presented for virtually any in vitro analysis.