The requirement of ATP and extra-ribosomal proteins for the inactivation of ribosomes by eight plant RNA N-glycosidases [ribosome-inactivating proteins (RIPs)] was investigated. Tritin, pokeweed antiviral protein and barley RIP depend, as gelonin [Sperti, S., Brigotti, M., Zamboni, M., Carnicelli, D. and Montanaro, L. (1991) Biochem. J., 277, 281-284], on the presence of ATP and extra-ribosomal proteins for full inactivation of ribosomes, while bryodin, lychnin, momordin, momorcochin and saporin inactivate isolated Artemia salina ribosomes suspended in buffer saline.