The phagocytic function of monocytes was measured by complement-binding zymosan particles (ZC). The phagocytic indices of monocytes prepared by glass-adherence and by gradient centrifugation were closely correlated (r=0.95, p<0.01). The former method was easier than the latter in the identification of monocytes. The phagocytic index was significantly reduced in patients with bronchogenic carcinoma (mean=53.2±20.2, p<0.01), malignant lymphoma (mean=52.9±20.6, p<0.01), and sarcoidosis (mean=51.1±20.2, p<0.01) compared with controls (mean=76.0±14.5). In contrast, there was no significant difference between controls and patients with pulmonary tuberculosis (mean=77.0±14.2). In bronchogenic carcinoma patients, a low phagocytic index was observed in all clinical stages. However, there was no significant correlation among phagocytic index, histological type and stage of disease. The C3 and Fc receptors of monocytes were also determined by rosette formation with ZC and IgG-treated ox erythrocytes (EA), respectively. In bronchogenic carcinoma, the percent of C3 receptor bearing monocytes was lower than that of controls, but the percent of Fc receptor bearing monocytes was in the normal range. However, the phagocytosis of EA was enhanced in comparison with controls. These data showed that monocyte phagocytic function was suppressed in patients with bronchogenic carcinoma. It was suggested that malignant tumors might affect monocyte function.