Three ribosome-inactivating proteins (RIPs) similar to those already known (Stirpe et al. (1992) Bio/Technology 10, 405-412) were purified from the seeds of Phytolacca dioica. These proteins, called Phytolacca dioica RIPs (PD-S1, PD-S2 and PD-S3 RIPs), are glycoproteins, with M(r) approx. 30,000, inhibit protein synthesis by a rabbit reticulocyte lysate and phenylalanine polymerization by isolated ribosomes, and depurinate rat liver rRNA in an apparently identical manner as the A-chain of ricin and other RIPs (Endo et al. (1987) J. Biol. Chem. 262, 5908-5912). Part of the purified rat liver ribosomes appeared resistant to the action of PD-S RIPs. The most abundant protein, PD-S2 RIP, gave a weak or nil cross-reaction with sera against various other RIPs, including a pokeweed antiviral protein from the roots of Phytolacca americana. PD-S2 RIP was linked to a monoclonal antibody (Ber-H2) against the CD30 human lymphocyte antigen and the resulting immunotoxin was selectively toxic to the CD30 + Hodgkin's lymphoma-derived L540 cell line.