Abstract Ribosome-inactivating proteins (RIPs, review by Barbieri et a/. 1993) are a class of proteins present in various tissues of several plants which inactivate mammalian ribosomes and, with less activity and to variable extent, plant, fungal, and bacterial ribosomes. They are enzymes, N-glycosidases, which release adenine from rRNA. They are divided into two groups: type 1, consisting of a single peptide chain, and type 2, in which an A chain with enzymatic activity is linked to a slightly larger B chain with the properties of a galactosyl-specific lectin. The B chain binds to galactosylterminated receptors on the membrane of most animal cells, allowing the molecule to enter the cytoplasm. Thus most type 2 RIPs are potent toxins, ricin being the best known, although recently some nontoxic lectins with the structure of type 2 RIPs have been described. Type 1 RIPs do not enter easily into cells, and consequently are much less toxic than type 2 RIPs.