Ricin B chain has been isolated from highly purified RCA II. B chain is the component of ricin which binds surface carbohydrate receptors and facilitates the transport of ricin into the cell. Although our ricin B chain has a substantially lower toxicity than native ricin in both cell culture test systems and cell-free protein synthesis assays, essentially all of the galactose/N-acetylgalactosamine binding activity is retained.