The chemotactic responsiveness of peripheral blood monocytes was measured in 71 patients with bronchogenic carcinoma, 13 patients with sarcoidosis, 14 patients with pulmonary tuberculosis and 91 normal controls. The monocyte chemotactic response (MCR) was significantly depressed in patients with bronchogenic carcinoma (mean=19.6±6.6,P<0.001) as compared to normal controls (mean=33.7±7.4). On the other hand, MCR was significantly enhanced in patients with pulmonary tuberculosis (mean=41.6±7.9,P<0.01). There was no significant difference from normal controls in patients with sarcoidosis (mean=27.5±8.5). In bronchogenic carcinoma patients, depression of the MCR was observed in all clinical stages. The MCR was further depressed in patients with evidence of distant metastasis compared to those without metastasis. There was no significant correlation between the MCR and histologic types of tumors. The MCR was significantly lowered by combination chemotherapy in 21 cases who received chemotherapy, but was elevated in 5 of 6 patients who had a remission after combination chemotherapy. These data showed that monocyte function in patients with bronchogenic carcinoma was suppressed, and suggested that malignant tumors themselves might affect monocyte function.