Two potent antimyeloid immunotoxins (IT) were generated by conjugating AML-2-23 (anti-CD14) and MCS-2 (anti-CD 13) monoclonal antibodies (mAb) to the ribosome-inactivating phytotoxin, ricin. Both IT selectively bound to target cells, inhibited protein synthesis, and prevented the clonogenic growth of fresh marrow blasts from acute nonlymphocytic leukemia patients as well as KG-1 (ANLL) cells. Cryopreservation did not inhibit IT activity. We conclude that AML-2-23-ricin and MCS-2- ricin show potential for effective ex vivo marrow purging in autologous bone marrow transplantation (ABMT) for ANLL. To our knowledge, this study represents the first evidence of the clinical potential of IT in high-risk ANLL.