Ivermectin delays tumor growth and reduces tumor mass in leukemia mouse xenografts. Human leukemia (OCI-AML2 and K562) and murine leukemia (MDAY-D2) cells were injected subcutaneously into the flank of sublethally irradiated NOD/SCID mice. Four (OCI-AML2), 5 (MDAY-D2), or 7 (K562) days after injection, once tumors were palpable, mice were treated with ivermectin (IVM; 3 mg/kg) by oral gavage in water or vehicle control (n = 10 per group) for 10 days (K562) or 8 doses over 10 days (OCI-AML2). MDAY-D2 mice (n = 10 per group) were treated with escalating doses of ivermectin (3 mg/kg for 4 days, 5 mg/kg for 3 days, and then 6 mg/kg for 3 days). Fourteen (OCI-AML2), 15 (MDAY-D2), or 17 (K562) days after injection of cells, mice were killed, tumors were excised, and the volume and mass of the tumors were measured. The tumor weight and the mean volume ± SEM are shown. Differences in tumor volume and weight were analyzed by an unpaired t test. ***P < .0001.