Monoclonal antibodies designated "C6" and "A5" identify cell surface carbohydrates shared by embryonal carcinoma cells and early mouse embryos. The binding of both antibodies to F9 embryonal carcinoma cells was inhibited by N-acetyllactosamine. While antibody C6 did not agglutinate human erythrocytes, antibody A5 agglutinated adult, but not fetal, erythrocytes of both type A and O, suggesting partial specificity for branched polylactosamine structures. Antibodies C6 and A5 did not label preimplantation stage embryos; however, labeling with both antibodies was observed following treatment of embryos with neuraminidase. In paraffin sections of postimplantation stage embryos, C6 and A5 exhibited similar yet distinct patterns of labeling, restricted primarily to the luminal surfaces of ectodermal and visceral endodermal epithelia. Neuraminidase treatment was found to expose additional patterns of C6 and A5 labeling within the ectoderm and mesoderm of the postimplantation embryo, not restricted to periluminal surfaces. These results suggest that cell surface carbohydrates are modified during early embryogenesis, in part, by selective patterns of sialylation.