StarD7 protein forms stable Gibbs and Langmuir monolayers at the air-buffer interface showing marked surface activity. The latter is enhanced by penetration into phospholipid films at an initial surface pressure above the protein's own equilibrium adsorption surface pressure to a lipid-free interface. The protein-phospholipid stabilizing interactions at the interface depend on the lipid, with preference for phosphatidylserine, cholesterol, and phosphatidylglycerol, and the increases of lateral surface pressure generated are comparable to those of other membrane-active proteins. The surface activity of StarD7 is strong enough to thermodynamically drive and retain StarD7 at the lipid membrane interface where it may undergo lipid-dependent reorganization as indicated by changes of surface pressure and electrostatics. (C) 2003 Elsevier Inc. All rights reserved.