A monoclonal antibody, mAb6, was produced that specifically bound to chicken T lymphocytes. Immunofluorescence analysis using a fluorescence-activated cell sorter revealed that the antibody reacted with approximately 50% of blood lymphocytes and with approximately 40% of splenocytes and thymocytes. It did not react with bursal cells and erythrocytes. Among different types of hemopoietic cell lines, only a T cell line was reactive with mAb6. When coupled to Sepharose 4B beads, mAb6 was found to be highly mitogenic for chicken T cells. In soluble form, mAb6 inhibited concanavalin A-induced T cell proliferation and the cytotoxic activity of allosensitized T cells, the inhibition occurring in a dose-dependent manner for both assays. Thus, the tissue distribution and the effects of this antibody on T cell responses suggest that mAb6 recognizes a T cell receptor present on a large proportion of chicken T cells.