Cells of mouse embryos develop a polarization of microvillous distribution at compaction. Cells of the 4-cell embryo show a uniform pattern of fluorescent-ligand binding and an even distribution of microvilli. Each cell of the early 8-cell embryo has a uniform distribution both of microvilli and of fluorescent ligand. During the 8-cell stage, there is a progressive increase in the incidence of cells which show microvilli restricted to a region normally on the exposed surface of the embryo. When late 8-cell embryos were disaggregated to single cells, and these sorted by pattern of fluorescent-ligand binding, each of the four patterns of staining related consistently to a characteristic distribution of microvilli as viewed by scanning electron microscopy. The 16-cell embryo possessed an inside population of uniformly labelled cells with a sparse microvillous distribution, and an outside population of cells, each of which had a microvillous pole.