Objective: The preferred method for the purification of hematopoietic stem cells (HSCs), which can easily differentiate into blood cells, is the selection of CD34(+) cells by flow cytometry or magnetic discrimination. Methods are needed that effectively determine human stem cells without entirely relying on phenotypic cell surface markers. One of these methods is cytosolic aldehyde dehydrogenase (ALDH), which plays a role in retinoid metabolism and resistance of HSCs to alkylating agents such as cyclophosphamide. The prospective isolation of human hematopoietic stem and progenitor cells with different functions, as well as surface markers, are also recommended by ALDH and flow cytometry. In the above findings, ALDH(+) cells can be isolated from other cells and the most primitive hematopoietic stem cells can be isolated and used in clinical applications.