Cytotoxicity of lymphocytes against allogeneic target cells of a bronchogenic squamous cell carcinoma was examined by a microcytotoxicity test in lung cancer patients after the various kinds of anticancer treatment. Cytotoxicity in untreated cases at stages I, II, and III was higher than the range of controls, but cytotoxicity in those at stage IV decreased to the range of controls. Tumor resection augmented cytotoxicity in patients at stages II and III, when the assays were carried out 3 weeks after the operation. Local irradiation of tumors augmented cytotoxicity in patients at stages II, III, and IV, when the assays were carried out 1 week after the last irradiation. Immunotherapy with BCG cell-wall skeleton augmented cytotoxicity to a marked degree in patients at stages I, II, III, and IV, when the assays were carried out after 4 months of a continuous treatment. The microcytotoxicity test may be useful for estimating the reactivity of lymphocytes in lung cancer patients in various situations.