The human peripheral monocyte-mediated cytolytic acitivity against tumor cells derived from human squamous cell-type lung cancer was measured in 16 healthy donors, 24 patients with bronchogenic carcinoma, 6 patients with malignant lymphoma and 8 patients with sarcoidosis. The activity was significantly depressed in patients with bronchogenic carcinoma (p<0.05) and in patients with malignant lymphoma (p<0.01). However, there was no significant difference between normal controls and patients with sarcoidosis. There was no significant difference monocyte-mediated cytolytic activities according to clinical stage and histologic type in lung cancer patients. A statistically significant positive correlation between phogocytic acitivity and cytolytic activity of monocytes, between monocyte-mediated cytolytic activity and cytostatic activity was observed in lung cancer patients. The data suggest that monocyte-mediated cytolytic activity in tumor bearing hosts is suppressed, and this lower activity effects the immunological surveillance and host defence mechanism against malignant tumors.