The expression of receptors for N-acetylgalactosamine-recognizing lectins, namely Helix pomatia agglutinin (HPA), Sophora japonica agglutinin (SJA), Bauhinia purpurea agglutinin (BPA), Vicia villosa agglutinin (VVA), and Wistaria floribunda agglutinin (WFA) was studied in early mouse embryos and teratocarcinoma cells. Each of these lectins as well as Dolichos biflorus agglutinin (DBA) bound differently to early embryonic cells, with the exception of VVA and WFA which showed indistinguishable reactivities. SJA reacted intensely with visceral endoderm, but hardly at all with parietal and primitive endoderm. Therefore, SJA will be useful for analyzing the mechanism of visceral-endoderm formation. Furthermore, the inner cell mass (ICM) of early blastocysts reacted intensely with DBA, while the ICM of late blastocysts reacted only faintly with this lectin. Primary endoderm derived from the ICM reacted faintly with SJA, HPA, and DBA, and these reactivities increased again during the differentiation of the endoderm. Therefore, these three lectins could be used in the analysis of early stages during the differentiation of endoderm from the ICM. The results illustrate the highly complex nature of developmentally regulated alterations of cell-surface carbohydrates during the early stages of embryogenesis.