The structures of the Oligosaccharides present on a rat immunoglobulin E myeloma protein have been determined. The high-mannose-type structures are heterogenous with a variable number of mannose residues attached to an N,N′-diacetylchitobiose core. The smallest such structure was determined to be Manα1 → 6(Manα1 → 3)Manα1 → 6(Manα1 → 3)Manβ1 → GlcNAc → GlcNAc peptide. Larger structures contain additional mannose residues linked α1 → 2 to the terminal mannose residues. The spectrum of structures obtained was similar to that present at Asn 563 of human IgM. The complex-type chains are mainly biantennary chains with the structure: A reexamination of the oligosaccharides present on human IgE high mannose glycopeptide C-1 using digestion with endo-β-N-acetylglucosaminidases CII and D and αmannosidase suggest that their structures are like those on the rat IgE molecule rather than like the structure which had been previously proposed (J. Baenziger, S. Kornfeld and S. Kochwa, 1974, J. Biol. Chem.249, 1889-1896).