The chemotactic responsiveness of peripheral blood monocytes was studied before and after immunotherapy was administered to 56 patients with melanoma. Abnormal chemotaxis was found in 36 patients (64%) prior to treatment; this abnormality correlated with severity of disease and prognosis. Immunotherapy with BCG and sensitized autologous lymphocytes and X-irradiated melanoma cells or surgical removal of the neoplasm both reduced the percentage of patients with abnormal chemotactic responses. The best prognosis was found for those patients who had normal chemotaxis prior to therapy. The data support the hypothesis that abnormalities of monocyte function might render the host less likely to destroy developing neoplasms and that malignant tumors themselves might affect monocyte function.