Cancer stem cells (CSCs)/tumor initiating cells have been shown to exist in recent studies; however, it is challenging to isolate these cells. The latest evidence suggests that elevated aldehyde dehydrogenase (ALDH) activity is a hallmark of CSCs. In this study, mice implanted with Ehrlich ascites tumor (EAT) cells were used to isolate cancer stem cells. Femoral bone marrow aspirations were performed 15 days after the injection of EAT cells and Lin(-)ALDH(bright) and Lin(-)ALDH(low) cell populations were isolated. Lin(-)ALDH(bright) cells isolated from EAT-bearingmice accounted for 11.08 +/- 10.52% of all the Lin-cell population. Analysis of hematopoietic stem cell markers showed that Sca-1, c-kit, and CD38 were expressed higher in the Lin(-)ALDH(bright) population compared with Lin(-)ALDH(low). The Lin(-)ALDH(bright) population expressed P-glycoprotein, a product of the multidrug resistance (MDR) gene. P-gp activity measured by rhodamine 123 (Rh123) and blocked by verapamil. Among the cells treated with doxorubicin for 48 h, the Lin(-)ALDH(bright) cell groups were more resistant and had higher overexpression of Bcl-2 protein than Lin(-)ALDH(low).