Based on isolation, sequence determination and X-ray studies, the primary and three-dimensional structure of the glycoprotein mistletoe lectin I (MLI) are determined. ML-I is constituted of two chains (A chain: 254 amino acid residues; B chain: 264 amino acid residues) linked by a disulfide bridge. Three different structurally identified oligosaccharides (I, II, III) are attached to four Ntype glycosylation sites (NA112, NB61, NB96 and NB136). According to these structural characterizations, ML-I is a member of ribosome inactivating proteins (RIP) of type II. The three-dimensional X-ray structure allows a clear-cut picture of the highly toxic effects of ML-I caused by its RNA-N-glycosidase activity, which is in contrast to its immunomodulating activity, applied for the treatment of cancer patients.