Molecules carrying SSEA-1 were isolated from [3H]galactose-labeled embryonal carcinoma cells by detergent solubilization followed by indirect immunoprecipitation. The antigenic molecules were degraded by extensive pronase digestion or mild alkaline treatment, and the majority of the products thus formed were so large as to be excluded from a column of Sephadex G-50. Therefore, the major carbohydrate constituent of the antigenic molecule was embryoglycan, the glycoprotein-bound large glycan in early embryonic cells. Furthermore, the binding of isolated embryoglycan with anti-SSEA-1 was directly shown by a modified Farr's assay. From these results we concluded that SSEA-1 determinant was carried by the large glycan.