Monocytes from 21 patients with cancer of the lung and cancer of the prostate were studied prior to treatment. The absolute circulating monocyte count, serum lysozyme levels and monocyte IgG surface receptors were normal at all stages of the disease. Monocyte chemotaxis was defective in 45% of the patients. Serum chemotatic factor inactivator(s) that inhibit chemotaxis of normal monocytes were detected in 90% of the patients. In two of four patients the chemotactic factor(s) disappeared following surgical removal of localized tumors. The results of the chemotaxis studies may explain the data of defective delayed hypersensitivity reactions frequently seen in patients with malignancies. The defective chemotaxis and the presence of chemotatic factor inactivator(s) may interfere with the ability of monocytes to accumulate as macrophages in tumor sites.