The properties of aqueous suspensions of phospholipids, composed of concentric bimolecular lamellae, have been described and are similar to many of the important permeability properties of biological membranes. The structures thus resemble, in aqueous suspension, such membrane-bounded biological bodies as mitochondria, erythrocytes, or more closely, the “myelin-figure” form of lysosomes. The model membrane systems, when exposed to lytic or protective steroids, have been shown to undergo changes in permeability for simple cations in directions similar to biological membranes. These studies support the concept that they constitute valuable model systems, and furthermore suggest that the membrane-action of biologically active steroids result from their direct interaction with lipid, independent of polysaccharide, protein or active cell metabolism.