An immunotoxin comprised of an anti-B4 (anti-CD19) murine monoclonal antibody linked to the modified plant-derived toxin blocked ricin. The antibody moiety of anti-B4 blocked ricin immunotoxin binds to B lymphocytes that express B4; after internalization of the immunotoxin by the B4-expressing B cell, the ricin moiety cleaves the N-glycosidic bond between the ribose and adenine base at position 4324 in the B lymphocyte 28S ribosomal RNA, resulting in ribosome inactivation, inhibition of protein synthesis, and cell death. "Blocked" ricin is ricin which has been chemically modified such that the lectin binding sites of the B chain (galactose-binding sites) have been blocked by covalent attachment of affinity ligands, leaving the ribosome-inactivating activity of the ricin A chain intact.