Abstract: Using monocolonal antibodies OKT4 （inducer/helper）, OKT8 （supprssor/cytotoxic） T Lymphocyte subsets of peripheral blood and pleural effusion in 20 cases of carcinomatuous pleurisy and 15 cases of tuberculous pleurisy were examined by flow cytometry. The results showed that T% and T4/T8 ratio in pleural effusion and peripheral blood of patients with carcinomatous pleurisy were significantly lower than those in peripheral blood of helthy subjects （P＜0.01）; T4% in peripheral blood of patients with tuberculous pleurisy was slightely lower than that in peripheral blood of helthy subjects, but was still significantly higher than that in peripheral blood of patients with carcinomatous pleurisy （P＜0.01）; T4% and T4/T8 ratio in pleurisy effusion of patients with tuberculous pleurisy were significantly higher than those in pleural effusion of patients with carcinomatous pleurisy （P＜0.01）; and T4% also slightly higher than that in peripheral blood of the patients with tuberculous pleurisy itself and helthy subjects. All these showed that determination of T Lymphocytes subsets in pleural effusion is helpful in differntial diagnosis between carcinomatous and tuberculous pleurisy.