The monocyte count in the peripheral blood, chemotactic responsiveness (MCR) and chemotactic factor inactivator (CFI) were measured in 66 patients with gastric cancer. Monocyte counts in advanced gastric cancer patients were significantly increased. MCR was significantly depressed in gastric cancer patients in the advanced stage of the disease, as compared with findings in the control groups. Sixty-seven percent of the patients with stage IV carcinoma had abnormally depressed responses. The CFIs were significantly higher in gastric cancer patients than in the control groups, and there was a positive correlation between the degree of inhibition and extent of cancer progress. These data support the hypothesis that qualitative and quantitative abnormalities of circulating monocytes, as macrophage precursors, may hinder immunological host defense mechanisms.