The Bandeiraea simplicifolia lectin I (BSA-I) conjugated to fluorescein isothiocyanate was used as a histochemical reagent to study the mouse embryos from fertilization to early somitogenesis. No lectin binding could be detected on the embryonic cells in the preimplantation embryo. Lectin labeled intensely the zona pellucida. In the implanting embryos lectin binding was detected along the subtrophectodermal and Reichert's membrane, in the cytoplasm of the parietal and visceral endoderm, and the trophoblastic giant cells, but not in the ectodermal cells. Studies on explanted blastocyts cultured in vitro disclosed that the cytoplasmic BSA-I binding sites in trophoblastic cells develop gradually. In the 9-day somitic embryo BSA-I reacted with epithelial cells of the yolk sac, but not with the mesenchymal cells. A continuity between the lectin-reactive endoderm and the foregut epithelium could be demonstrated. These data indicated that BSA-I lectin can be used as a histochemical probe for endodermal (yolk sac) and trophoblastic differentiation in the peri-implantational mouse embryo.