The monocyte-mediated cytostatic acitivity against human tumor cells was measured in healthy donors and patients with bronchogenic carcinoma, malignant lymphoma, sarcoidosis and pulmonary tuberculosis. The target cells employed were HeLa cells and squamous cell-type lung cancer derived EBC-1 cells. Cytostatic activity was measured by inhibition of (3)H-thymidine uptake by target cells. The mean value of monocyte-mediated cytostasis in HeLa cells was 46.4±11.8% in 18 healthy donors, 47.7±19.0% in 31 patients with bronchogenic carcinoma, 39.9±21.7% in 19 patients with malignant lymphoma, 46.2±17.7% in 14 patients with sarcoidosis, and 48.6±15.7% in 9 patients with pulmonary tuberculosis. The mean value of monocyte-mediated cytostasis in EBC-1 cells was 87.4±8.5 % in 18 healthy donors, 78.9±16.8% in 39 patients with bronchogenic carcinoma, 82.5±15.1% in 25 patients with malignant lymphoma, 87.0±6.7% in 16 patients with sarcoidosis, and 94.3±8.8% in 8 patients with pulmonary tuberculosis. No significant difference between healthy donors and patients with the above mentioned disorders was observed in the HeLa cell assay. But there were remarkably low activities, two standard deviations below the mean value of healthy donors, in 30% of the patients with bronchogenic carcinoma and 14% of the patients with malignant lymphoma in the EBC-1 cell assay. In lung cancer patients no significant positive correlation was observed between monocyte-mediated cytostatic acitivity and other monocyte functions. The data suggest that the tumor cell killing activity of peripheral monocytes was decreased in patients with malignant neoplasma.