The majority of malignant myeloid progenitor cells express receptors for GM-CSF. The fusion of GM-CSF with diphtheria toxin allows a targeting of cells with GM-CSF receptors for effects of the toxin while sparing GM-CSF receptor-lacking multipotent stem cells. The great majority of AML cells express GM-CSF receptors and DT388GMCSF has shown selective killing of AML and CMML progenitors in vitro while sparing normal progenitor cells. When administered as a single bolus to rodents, adequate blood DT388GMCSF biological activity was found to kill several logs of leukemic cells. A phase I clinical trial of DT388GMCSF given as a daily bolus i.v. infusion for up to 5 consecutive days was completed in 38 patients. The study defined liver toxicity as the DLT. The liver toxicity was observed only in patients > 50 years and receiving steroids. Responses were seen in four patients consisting of one complete remission and 3 partial remission of short duration. Peak drug levels were inversely proportional to pre-treatment DT388GMCSF antibody levels.