The novel fusion protein DT(388)IL3, composed of the catalytic and translocation domains of diphtheria toxin (DT(388)) fused with a Met-His linker to human interleukin 3 (IL-3), was tested for anti-leukemia efficacy in an in vivo model of differentiated human acute myeloid leukemia (AML). Six-week-old female SCID mice were irradiated with 350 cGy, inoculated 24 h later with 20 million (i.v., i.p., or s.c.) TF1 cells transfected with the v-SRC oncogene, and treated i.p., starting 24 h later, with up to five daily injections of saline, DT(388)IL3 (2 microg), DT(388)GMCSF (2 microg), DAB(389)IL2 (2 microg), or cytarabine (80 microg) or two weekly injections of anti-CD33-calicheamicin conjugate (5 microg). Animals were monitored twice daily, and moribund animals killed and necropsied. Control animals had a median disease-free survival (DFS) of 37 days (i.v., n = 45), 35 days (i.p., n = 20), and 21 days (s.c., n = 20), respectively. Only 5/49 (10%) of the DT(388)IL3 treated i.v. inoculated animals died with leukemia. Median DFS with i.v., i.p. and s.c. tumor inoculated animals was prolonged by fusion protein treatment to >120 days, 66 days and 31 days (P < 0.001, = 0.0003, and = 0.0006), respectively. Median DFS with s.c. tumor inoculated animals was also prolonged by other active anti-leukemia agents (DT(388)GMCSF, cytarabine and anti-CD33-calicheamicin) relative to controls by 67%, 172% and 47% (P < 0.0001, <0.0001, and =0.0004), respectively. In contrast, median DFS with s.c. tumor inoculated animals treated with DAB(389)IL2 non-significantly reduced by 13% relative to controls (P = 0.21). Thus, DT(388)IL3 fusion protein demonstrates in vivo anti-leukemia efficacy and warrants further preclinical development for treatment of chemo-resistant, IL-3 receptor positive AML patients.