the present invention is directed to the use of agents that induce high levels of cell surface molecules to provide targets for immunotoxins directed against the same cell surface molecules . a specific example is given in which all - trans - retinoic acid is used to induce high levels of cd38 cell surface antigen expression in several myeloid and lymphoid leukemia cells . cd38 was then used as target for delivering plant toxin to leukemia cells . treatment of leukemia cells with ra induced high levels of cd38 in those cells that otherwise had low cd38 expression . the ra - induced leukemia cells then became exquisitely sensitive to an immunotoxin constructed from an anti - cd38 monoclonal antibody conjugated to the plant toxin gelonin .