Immunotoxins are conjugates between antibodies especially directed against cancer cells and a subunit of a powerful toxin. We used the A-chain of ricin. These conjugates are specifically cytotoxic when used at very low concentrations in vitro and can destroy more than 99.99% of clonogenic cells. The efficacy of immunotoxins was also demonstrated in vivo but is inferior to its in vitro potency. For this reason the first use of immunotoxins in man can be the cleaning up of bone marrow from leukemic cells in the near future.