Treg cells from colorectal cancer tissues are functional and suppress autologous CD25- lymphocytes. Tumour infiltrating Treg cells suppress the proliferation of autologous tumour infiltrating CD4+ CD25- T cells (n = 3). Responder CD4+ CD25- T cells were cultured either alone (left bar) or with different ratios of autologous CD4+ CD25+ Treg cells isolated from the tumour, in the presence of anti-CD3 and anti-CD28 antibodies. Proliferation was assessed by ³H incorporation after 5 days of culture.