A glycosylated form of recombinant human insulin-like growth factor I (IGF-I) expressed in Saccharomyces cerevisiae was shown to contain mannose as the only carbohydrate constituent. All oligosaccharide chains of the glycoprotein could be released by mild alkaline treatment, and separated from the protein by gel-permeation chromatography on Bio-Gel P-2. The structures of these O-linked carbohydrate chains were determined by 500-MHz 1H-NMR spectroscopy, affording the disaccharide Man alpha 1-2Man as the major component and the tetrasaccharide Man alpha 1-3Man alpha 1-2Man alpha 1-2Man as a minor component. Reference oligosaccharides were prepared from mannoproteins released from the cell wall of S. cerevisiae X2180 (alpha-wild type). In addition to previously reported structures, ranging from mannose to mannotetraose, the pentasaccharide Man alpha 1-3Man alpha 1-3Man alpha 1-2Man alpha 1-2Man was identified in the cell wall mannoprotein.