Ribosome-inactivating proteins are a group of closely related proteins that are widely distributed throughout the plant kingdom and which share the unusual property of being able to inactivate mammalian ribosomes by an enzymic (non-stoichiometric) mechanism (1). Two major classes of these proteins are found in plants. The first contains the plant toxins (ricin, abrin, modeccin) in which an enzymically active Achain is linked through a disulfide bond to a cell-binding B-chain (2). These toxins bind to and penetrate mammalian cells. This is followed by release of the A-chains into the cytosol where they inactivate ribosomes and kill the cells. The second class of ribosome-inactivating proteins (RIPs) consists of proteins that are single polypeptide chains and which resemble the A-chains of plant toxins in their physical-chemical and enzymic properties (1, 3), as well as in their amino-terminal amino acid sequences (4). These proteins are relatively non-toxic, for they lack a B-chain and bind poorly to mammalian cells. However, they do show significant toxicity to phagocytic cells (1) and to virus=infected cells (1, 3) where the virus apparently helps the protein penetrate the cell. This review will consider only this second class of "non-toxic" RIPs. In contrast to the relatively rare occurrence of plant toxins, "non-toxic" RIPs are widely distributed throughout the plant kingdom. In their excellent review, Barbieri and Stirpe (1) catalogue RIPs from 51 plant species representing 14 different families. Screening surveys have identified other plant species without apparent RIP activity (5 7), but it is possible that if more appropriate assay conditions were selected most, even all, plant species would be found to contain RIPs (8). The 51 RIPs were obtained from seeds, roots, leaves, and/or sap of plants (1, 9, 10), and were frequently present at very high concentrations, e.g., 2-300mg protein/100g of plant tissue (1). Before the