Abstract: The invention describes specific sialylated structures present on human stem cells and cell populations derived thereof. The invention is especially directed to methods to control the status of stem cells by observing changes in sialylation of the cells; and control of potential contaminations of biological materials; and reagents and methods used in connection with the cells in order to avoid alterations of the cell glycosylation by contaminating materials. The invention is further directed to novel stem cells, the glycosylation of which has been specifically altered.