The F0F1 ATP synthase of chloroplasts was imaged using atomic force microscopy (AFM) in contact mode under physiological conditions. Chloroplast (CF0F1) ATP synthases were reconstituted into liposomes, Liposomes were adsorbed on a mica surface where they spread and formed lipid bilayers containing CF0F1 ATP synthases which could be imaged. From these reconstituted CF0F1 ATP synthases, the CF1 part could be removed either by application of a chemical denaturant or less efficiently by mechanical stripping with the AFM tip. Embedded in the lipid bilayer were seen ring-like structures with a central dimple with outer diameters of 20 +/- 3 nm (chemical denaturant) and ca, 7 nm (mechanical stripping), respectively, Ring-like structures were also observed in a protein-free lipid bilayer, These had diameters of 30 +/-5 nm and could be clearly distinguished from the structures observed after mechanical stripping, Hence, the ring-like structures observed after mechanical stripping might represent the intrinsic membrane domain CF0 or the oligomer of its subunit III, In addition, isolated CF1 adsorbed directly onto the mica surface was imaged, In accordance with the size known from electron microscopy, a diameter of 13 +/- 4 nm was measured, (C) 1997 Academic Press.