We have surveyed 14 different toxic and nontoxic ribosome-inactivating proteins from plants for the ability to act on the RNA of the eucaryotic 60 S ribosomal subunit. All of these proteins act to introduce a specific modification into 26-28 S RNA which renders the RNA sensitive to cleavage by aniline. Sequence analysis of the 5'-termini of the fragments produced by ricin and saporin following aniline cleavage indicate that both proteins possess identical specificity. Our observations support the conclusion of Endo and Tsurugi (J. Biol. Chem. 262, 8128-8130, 1987) that ricin is a specific N-glycosidase and we have located the site of this cleavage by direct sequence analysis. Our results further suggest that all plant ribosome-inactivating proteins function as specific N-glycosidases with the same specificity.