Apolipoprotein C-III 1 and apolipoprotein C-III 2 each contain one oligosaccharide side chain, bound O-glycosidically to threonine in position 74 of the amino acid sequence. The studies reported in this paper characterize these alkali labile oligosaccharides, thereby demonstrating the complete structure of apolipoprotein C-III. Monosaccharide analysis revealed the following sugar composition: D-galactose/N-acetyl-D-galactosamine/sialic acid 1: 1: 1 and 1: 1: 2 for apolipoprotein C-III 1 and apolipoprotein C-III 2, respectively. Treatment of desialylated apolipoproteins with alkaline borohydride released the reduced disaccharide β-D-galactosyl-(1→ 3)-N-acetyl-D-galactosaminitol, which was detected by gas-liquid chromatography. Further studies employing periodate oxidation and Smith degradation indicated that the structure of the trisaccharide from apolipoprotein C-III 1 was α-N-acetylneuraminyl-(2→ 3)-β-D-galactosyl-(1→ 3)-N-acetyl-D-galactosaminitol. The tetrasaccharide structure from apolipoprotein C-III 2 is made up of this trisaccharide plus one sialic acid residue linked to C 6 of N-acetyl-D-galactosaminitol, as was shown by the assessment of chromogens formed upon alkaline degradation.