AbstractIn this report we describe a new method of immobilizing intact mouse preimplantation embryos and their dissociated blastomeres on a flat substrate so that they may be processed en masse for electron microscopy. The embryos can be arranged so that they will be coplanar and grouped together closely enough to be included within the same plane of section. The substrate consists of Thermonex plastic coverslips that have been coated with a layer of fibroblasts or with fibronectin. The plant lectin phytohemaggiutinin (PHA) is used as a ligand to adhere the embryos onto the coverslips. This method is compatible with other ligands and with a variety of immunocytochemical procedures and can be used with live or fixed embryos.