Levamisole, a potentiator of cell-mediated immunity, has been reported to increase survival in patients with resectable carcinoma of the lung. Cell-mediated immunity can be measured in vitro by the leukocyte migration inhibition test. We have previously reported that this test detects cell-mediated immunity to human lung tumor antigens. In the present studies, patients with lung cancer were treated with Levamisole. Their leukocytes were evaluated in the leukocyte migration inhibition assay before, during, and following Levamisole therapy. Small increases in cell-mediated immunity were observed when patients had a high pre-existing tumor immunity. When tumor-associated reactivity was absent prior to therapy, larger increases were measured. Although Levamisole is a nonspecific immunostimulant, these data indicate that in vitro anti-tumor immune responses are enhanced by Levamisole therapy. Augmentation of cell-mediated immunity to tumor antigens may explain the clinical benefits of Levamisole therapy.