Over the last years regulatory T cells (Tregs) were defined as CD4(+)CD25(+) T lymphocytes expressing the transcription factor Foxp3 (Forkhead Box P3) with the ability to downregulate various immune responses. In swine the existence of CD4(+)CD25(+) T lymphocytes was described before but nothing is known about the function of this minor cell population to date. Therefore, we studied porcine CD4(+)CD25(+) T cells with regard to major attributes of murine and human Tregs: their phenotype concerning the expression of several T-cell specific antigens, including Foxp3, their IL-10 production and their suppressive capacity. Our results revealed that porcine CD4(+)CD25(+) T cells with high CD25 expression count for about 2-9% of the CD4(+) T-cell subset. They demonstrate a strong Foxp3 expression, a heterogeneous CD45RC-, CD8alpha- and MHC-II-defined phenotype and a moderate IL-10 production. Co-cultivation of increasing numbers of CD4(+)CD25(high) T cells with a constant number of CD4(+)CD25(-) responder-T cells caused a decrease in proliferation of the entire culture. This demonstrates the suppressive capacity of the CD4(+)CD25(high) T-cell subset and - together with their Foxp3 expression - the existence of porcine Tregs.