Normal mesenchymal stem cells (MSCs) inhibit the proliferation of leukemia MSCs. MSCs of umbilical cord blood and normal bone marrow and leukemia MSCs were isolated and cultured according to our previous report. The second passage leukemia MSCs stained with CMDil (1×104) were cultured for 24 h, then the MSCs of umbilical cord blood and bone marrow were co-cultured with the leukemia MSCs with the ratio of 1:1, 10:1 and 100:1, respectively. After five days, the numbers of leukemia MSCs were detected by flow cytometry. The inhibition rate of leukemia MSCs was calculated according to the following: inhibition rate = (numbers before co-culture - numbers after co-culture) / numbers before co-culture. The inhibition rate of the MSCs from umbilical cord blood was significantly higher than that of the MSCs from normal BM. The inhibition rate increased as the number of umbilical cord MSCs increased, but decreased as the number of BM-MSCs increased. * p<0.01.