Investigation of lateral heterogeneities (clusters) in cell membranes is an important step toward understanding the physical processes that lead to the formation of lipid domains and rafts. Computer model- ing methods represent a powerful tool to solve the problem, since they can detect clusters containing only a few lipid molecules-the situation that still resists characterization with modern experimental techniques. Parameters of clustering depend on lipid composition of a membrane. In this work, we propose a computa- tional method to detect and analyze parts of membrane with different packing densities. Series of one- and two-component fluid systems containing lipids with the same polar heads and different acyl chains, dioleoylphos- phatidylcholine (18 : 1) and dipalmitoylphosphatidylcholine (16 : 0), were chosen as the objects under study. The developed algorithm is based on molecular dynamics simulation of hydrated lipid bilayers in all-atom mode. The method is universal and could be applied to any other membrane system with arbitrary lipid com- position. Here, we demonstrated that the studied lipid bilayers reveal small lateral dynamic clusters composed of just several (most often, three) lipid molecules. This seems to be one of the most important reasons deter- mining the "mosaic" nature of the membrane-water interface. © Pleiades Publishing, Ltd., 2011. © Pleiades Publishing, Ltd., 2011.