The distribution along the polypeptide of the carbohydrate units of two major calf thyroid cell surface glycoproteins, GP-1 and GP-3, was obtained from a study of their glycopeptides obtained after Pronase digestion. The GP-3 molecule (Mr = 20,000) yielded two large glycopeptides (Mr = 9,500 and 7,000) in equimolar amounts which each consisted of one N-linked (Mr = 5,400) and several small O-linked oligosaccharides accounting for a total of nine carbohydrate attachment sites in a 27-amino acid residue segment of the peptide chain. The Pronase treatment of GP-1 (Mr = 100,000) revealed the presence of a large protease-resistant fragment (Mr = 50,000) which contained 34 carbohydrate units (eight N-linked and 26 O-linked) in a segment of 105 amino acids. In addition to these densely glycosylated peptides (one glycosylation site/3 amino acid residues), small glycopeptides with polymannose saccharide units were found in the digests of both proteins. The occurrence of repeating N-acetyllactosamine sequences in the N-linked carbohydrate units of GP-1 and GP-3 was suggested by the composition and size of the oligosaccharides released by hydrazinolysis and was demonstrated by endo-beta-galactosidase treatment. The cleavage products from digestion with this enzyme were identified as NeuAc alpha 2----6Gal beta 1----4GlcNAc beta 1----3Gal, Gal alpha 1----3Gal beta 1----4GlcNAc beta 1----3Gal, Gal beta 1----4GlcNAc beta 1----3Gal, and GlcNAc beta 1----3Gal with the tetrasaccharides constituting the predominant species. The terminal alpha-D-Gal residues accounted for the binding of GP-1 and GP-3 glycopeptides to Bandeiraea simplicifolia I-agarose; concanavalin A-Sepharose affinity chromatography indicated that most of the N-linked carbohydrate units of both glycoproteins contained more than two branches. Difference in the branching on the poly-N-acetyllactosamine sequences of GP-1 and GP-3 was suggested by the finding that only the latter glycoprotein, as well as its glycopeptides, reacted with anti-blood group I antibodies; neither glycoprotein demonstrated blood group i antigenicity. Examination of cultured thyroid follicular cells revealed that both I and i determinants were present at the cell surface.