For isolation of CD4+ CD25+ regulatory T cells from peripheral blood mononuclear cells. The kit includes all Fab Streptamer components for double positive selection of cell subsets with magnetic microbeads* and for removal of Streptamers from isolated cells. 1.) Strep-Tactin(R) Magnetic Microbeads for Fab Streptamers; 3 vials for 5x10^8 cells each 2.) CD4 Fab-Strep human for isolation of T-cells; 1 vial for 1x10^9 cells 3. ) CD25 Fab-Strep human for isolation of T-cells; 1 vial for 5x10^8 cells 4. ) D-Biotin stock solution for Streptamer(R) technology; 3x 1 ml 5.) Buffer IS, Streptamer(R) Washing Buffer, 10x concentrated; 2x 50 ml This product requires dry ice shipment and is shipped directly from our Headquarters in Germany. *Microbeads are intended for use with a permanent magnet for manual cell isolation; we recommend our StrepMan magnet for 2 x 15 ml and 2 x 50 ml Falcon tubes. Principle of Selection: Isolation of human CD4+ CD25+ regulatory T cells is performed by serial double positive selection. In a first step PBMCs are labeled with magnetic CD4 Fab Streptamers. Labeled cells are isolated in a strong magnet (StrepMan, horizontal position) where they migrate toward the tube wall on the side of the magnet. This CD4 positive cell fraction is collected and cells are liberated from all labeling reagents by addition of biotin in a strong magnet (StrepMan, upright position). The magnetic Streptamers migrate toward the tube wall and the label-free cells remain in the supernatant. Biotin is removed by washing. The pure and label-free CD4 positive cells are subjected to a second positive selection with magnetic CD25 Fab Streptamers following the same procedure. The resulting cell preparation is highly enriched with CD4+ CD25+ regulatory T cells with a purity of more than 90%. No depletion steps and no columns are needed.