BACKGROUND: Cytokine response changes with ageing. CD26 is known as an operational marker for Th1 pathway. In this study we investigated the possibility of CD26 as a practical marker for immunosenescence, Th1 or Th2 polarization. METHODS: We studied 99 subjects from 4 groups divided by age (thirties and sixties) and by hepatitis B. CD26 expression in T cells in peripheral blood was investigated by 3 color flow cytometry. Especially, CD26 expression in activated (HLA-DR+) T cells was compared among each group. RESULTS: CD26 expression in total T cells had the trend toward a decrease with ageing and decreased significantly especially in Ts cells (from 45.9+/-11.3% to 37.5+/-12.4%, P<0.05). Regardless of hepatitis B, CD26 expression in activated Th cells was not significantly different between the two age groups. HLA-DR expression in total T cells had the trend toward an increase in the elderly or hepatitis B and the dominant T cell subset responsible for this increase was Ts cell subset in the elderly and was Th cell subset in hepatitis B. CONCLUSIONS: We could not find the possibility of CD26 as a practical marker for immunosenescence reflecting Th1 polarization in activated Th cells. But CD26 or HLA-DR expression in total T cells had the trend to change with ageing and the change in Ts cells was more dominant than in Th cells.