The present invention relates to a method of identifying and separating non-regulatory T-cells (conventional T-cells) from a mixture comprising regulatory T-cells by using of the CD154 molecule (CD40 ligand) through depletion of CD154+ T-cells from the mixture or in combination with additional positive selection of Treg using markers that are specific for regulatory T-cells, such as for example, CD25, GITR, CTLA4 or markers which are specific for activated regulatory T-cells, such as, for example, CD137, “latent TGF-beta (LAP)”, GARP (LRRC32), CD121a / b, thereby generating a cell composition of activated Treg cells. The invention relates also to a kit comprising an antibody for detecting CD154 and at least one additional antibody for detecting markers for activated or non-activated regulatory T-cells. The antibodies can be coupled to a fluorescent dye or magnetic microparticles.