Abstract: A method for generating a population of functional regulatory T cells (TREG-cells), which are a subset of the T cell lineage having the ability to actively suppress immune activation and maintain peripheral immune tolerance, is described. The method comprises the steps of first culturing haemopoietic stem cells (HSC) and/or haemopoietic progenitor cells in the presence of a Notch ligand that supports T cell differentiation, and then isolating T cells having a TREG-cell surface marker phenotype. A suitable source of HSC is cord blood (CB) and a suitable culture medium is OP9 cells engineered to express the Notch Ligand Delta-Like 1 (DL1) (OP9-DL1 cell line). Examples of TREG-cell surface marker phenotypes are CD4+CD25+, CD45RO+, CD45RA+, CD127LOW?, LAG-3 and/or CD39+.