The development of the polarized surface binding of the fluoresceinated ligand concanavalin A (FITC Con A) was studied in blastomeres of the early mouse embryo. Single S-cell blastomeres, natural g-cell couplets derived from the in vitro division of individual 4-cell blastomeres, and reaggregated couplets made from dissociated S-cells were cultured for varying periods of time and on a variety of substrata. The development of surface polarity was found to be highly dependent upon cell contact. Over 50% of the cells in couplets were polarized after 4-5 hr in culture, with the smaller cell in the couplet usually more advanced in its polarization than the larger cell. The orientation of the poles of FITC-Con A binding was opposite the point of contact between cells in the couplets regardless of their previous orientation within the embryo or the plane of cleavage.