A recombinant protein consisting of human interleukin 3 (IL3) fused to the first 388 amino acids of diphtheria toxin [DT(388)] with potential antineoplastic activity. Upon administration, the IL3 moiety of the DT(388)IL3 fusion protein binds to IL3 receptors on cells expressing the receptor. Subsequently, the DT(388) toxin moiety, containing both translocation and catalytic domains, translocates across the cell membrane endocytotically; within the cytosol, the catalytic domain catalyzes the ADP-ribosylation of, and inactivates, translation elongation factor 2 (EF-2), resulting in the inhibition of translation during protein synthesis. IL3 may be overexpressed by acute myeloid leukemia (AML) cells.