CD26 is an activation antigen which is expressed on the surface of human T-lymphocytes. It has been characterized to be the dipeptidyl peptidase IV (DPP IV). Considerable amounts of CD26 are already present on resting T-lymphocytes. The expression of CD26 is enhanced by T-cell mitogens or antigens. A correlation of CD26 expression and of enhanced enzymatic activity was observed after T-cell activation. Our data indicate that not only the immunoreactivity, but also the enzymatic activity of CD26 are detectable on the cell surface. In addition, de novo expression of CD26 was demonstrated on CD26-negative T-cells after mitogenic or antigenic stimulation. CD26 expression is initiated during the G1 phase of the cell cycle. The expression occurs nearly simultaneously with HLA-DR, but later than CD25. Similar to CD25 and HLA-DR, CD26 is not a permanent marker on the surface of T-lymphocytes, but is down-regulated after 7 days of culture. When testing the influence of interleukin 1, interleukin 2, tumour necrosis factor, and interferon-gamma on the expression of CD26, no effect was found on unstimulated or on mitogen-stimulated T-lymphocytes. The binding of two different monoclonal antibodies against CD26 (anti-DPP IV and anti-Tal) to resting and activated T-lymphocytes revealed a different pattern of immunoreactivity. Resting T-lymphocytes reacted stronger with anti-DPP IV than with anti-Tal. However, binding of the two monoclonal antibodies to T-cell blasts did not show significant differences. These data indicate that CD26 may be expressed in differently modulated configurations on the surface of T-cells, which may be associated with a distinct status of activation and/or function.