Atomic force microscopy (AFM) has been used to investigate Langmuir-Blodgett films of the photosynthetic protein membrane obtained from photosynthetic bacteria Rhodopseudomonas viridis deposited on glass substrates. Membrane structures with small domains composed of hexagonally packed photoreaction units (PRUs) were imaged. The average spacing between PRUs in the AFM images is approximately 12 nm, which is in good agreement with transmission electron microscopy data. Observation on the 100 nm scale reveals that the films consist of membrane fragments of diameter ranging from 0.1 to 1 mum. The recently developed force modulation technique was also used to image the local mechanical properties of the samples. There was a large difference between the images of the protein and the substrate.