Saporin-S6, a ribosome-inactivating protein (RIP) from Saponaria officinalis released more than 1 mol of adenine/mol of ribosomes from house fly (Musca domestica) larvae and from rat liver. The release of adenine from rat liver ribosomes by several RIPs (plant enzymes with RNA N-glycosidase activity) was examined. Saporins, pokeweed antiviral protein from roots of Phytolacca americana (PAP-R), and trichokirin from Trichosanthes kirilowii seeds depurinated rat liver ribosomes at more than one site. Up to 33 mol of adenine were released from 1 mol of ribosomes. This property is not common to all RIPS. INTRODUCI ONRibosome-inactivating proteins (RIPs) from plants (reviewed in [11), of either type I (single-chain proteins) or type 2 (twochain proteins, ricin and related toxins), damage ribosomes from eukaryotes and, at higher concentrations, ribosomes from prokaryotes 12-4J. They are RNA N-glycosidases which depurinate the major ribosomal RNA at a position corresponding to adenine-4324 of rat liver 28 S rRNA [5,6]. This site of action is a highly conserved loop-and-stem structure, and the conditions necessary for its identification by ricin have been determined by Endo and co-workers [7]. This is the only known site of action of RIPs on eukaryotic ribosomes, although ricin [7] and Mirabilis antiviral protein [3] also depurinated naked 16 S Escherichia coli rRNA at another site, A-1014. We report here that saporins (RIPs from seeds, leaves and roots of Saponaria officinalis), pokeweed antiviral protein from Phytolacca americana roots (PAP-R), and trichokirin from Trichosanthes kirilowii seeds depurinate rat liver ribosomes more extensively than do other known RIPs.