BACKGROUND: It has been found that Helper T cells in the peripheral blood are decreased in the cell mediated immunity in the pulmonary tuberculosis But it has not been confirmed yet that only decrease in number of cells which has phenotype in the peripheral blood is defined to decrease in cell mediated immunity. The immunocytochemical study was performed to observe the change of the percentage of T-lymphocytes with their subsets and activated T cells in the peripheral blood of pulmonary tuberculosis and to know how many T cells would be activated, relative to resting cells in the peripheral blood. METHODS: The peripheral blood obtained from twenty two patients and ten healthy controls were smeared on the gelatin coated slide glass prepared for of mononuclear cells. The double bridge technique of alkaline phosphatase-antialkaline phosphatase(APAAP) method was used. As the primary antibodies, T1(anti-human T cell), T4(anti-human helper/inducer T cells) and T8(anti-human supressor/cytotoxic T cell) antibodies and interleukin-2 receptor (for early activated T cell),very late activation antigen (for activated cytotoxic T cell), T cell lineage specific activation antigen monoclonal actibodies were used. RESULTS: 1) There were significantly decrease in the absolute number of T4(+) cells but significantly increase of T8(+) cells in the peripheral blood of pulmonary tuberculosis (p<0.05). 2) The percentage of T4(+) cells showed significantly decrease in pulmonary tuberculosis but T8 (+)cells significantly increase(p<0.05). T4(+)/T8 (+) ratio showed significantly decrease in the peripheral blood of the pulmonary tuberculosis(p <0.05) 3) There were significantly increase in the absolute number of variable stages of activated T cells in the peripheral blood of the pulmonary tuberculosis(p<0.05). 4) The percentage of IL-2R, VLA-1, TLiSA were 6.45+1.56%, 7.64+1.34*, 10.45+1.16% in order which showed significantly increase in the peripheral blood of the pulmonary tuberculosis(p <0.05). CONCLUSION: We speculate that only a few percentage of T lymphocyte is activated in cell mediated immunity in pulmonary tuberculosis.