The crystal structure of pokeweed antiviral protein from seeds of Phytolacca americana (PAP-S) was solved at 1.8A. PAP-S is a one-chain ribosome-inactivating protein (RIP) and distinctively contains three well-defined N-acetylglucosamines, each covalently linked to an asparagine residue at positions, 10, 44, and 255, respectively. The high-resolution structure clearly shows the three mono-sugars to have either an alpha- or a beta-conformation. Two of sugars are located on the same side of the molecule with the active pocket. Except one hydrogen bond, there are no intermolecular interactions between the polypeptide chain and the sugars. Instead the sugar conformations appear to be stabilized by intermolecular interactions. The sugar structure defined at high resolution provides a structural basis for understanding their possible biological activity. The structural comparisons of PAP-S with other PAPs reveal that the major disparity of these homologous molecules is the different charge distribution on the upper right side of the front side near the active pocket. Based on the available structure of the 50S ribosomal subunit, the possible interactions between PAPs and the ribosome are discussed.