The subcutaneous injection of cells of any one of five unselected murine tumors resulted very rapidly in the liberation into the circulation of a small molecular weight factor that severely impaired the capacity of the host to resist experimental infection with Listeria monocytogenes and Yersinia enterocolitica. It was found that the factor appeared in blood within 8 h of injecting tumor cells subcutaneously. That it possessed potent physiological activity was evidenced by the demonstration that an infusion of as little as 0.015 ml of tumor-bearer serum strikingly suppressed the capacity of normal recipients to resist bacterial infection. It was reasoned on the basis of the knowledge that the only cells in mice with the capacity to destroy Listeria are macrophages, that suppression of antibacterial resistance was caused by the ability of the tumor-suppressor factor to interfere, either directly or indirectly, with the antibacterial functions of these mononuclear phagocytic cells. The results are consistent with the hypothesis that at least some malignant neoplastic cells are naturally selected to avoid destruction by native and acquired antitumor mechanisms of mononuclear phagocytes.