Peripheral blood Treg cells in colorectal cancer patients are functional and suppress proliferation of autologous CD4+ CD25- lymphocytes. Stimulated Treg cells from colorectal cancer patients suppress the proliferation of autologous PBL CD4+ CD25- T cells (n = 3) in a dose-dependent manner. Responder CD4+ CD25- T cells were cultured either alone (left bar) or with different ratios of autologous CD4+ CD25+ Treg cells in the presence of anti-CD3 and anti-CD28 antibodies. Proliferation was assessed by ³H incorporation after 5 days of culture.