We report in this study the topography of a single layer of Cyt f and of a mixture of MGDG/Cyt f as a function of surface pressure (Pi) and concentration of spreading solvent (ethanol). We used atomic force microscopy (AFM) to image the Langmuir-Blodgett (LB) films transferred from the air/water interface onto freshly cleaved mica. We observed that pure Cyt f tends to form assemblies with an average width of 12 nm at low ethanol concentration (20%), and domains about 100 nm wide at higher concentration (80%). However, in the mixed LB films (MGDG/Cyt f), the protein appear in the monomer form with a lateral dimension of similar to 3 nm. From these results, we consider that Cyt f assemblies are formed by an octamer.