Cytotoxicity of peripheral blood lymphocytes against a cultured cell line QG-56 of a bronchogenic squamous cell carcinoma was studied by a microcytotoxicity test in 33 cases of bronchogenic carcinoma. An augmented cytotoxicity was detected in the patients in early stages of the disease as compared to normal controls or the patients carrying other types of tumors. The cytotoxicity declined as the stages of cancer advanced. The patients with high degree of cytotoxicity appeared to show better clinical course after radiotherapy.