Mice carrying any one of three murine tumors in their right hind foot pad were incapable of eliminating an inoculum of the bacterial parasite Listeria monocytogenes from the progressive tumor. In contrast, they were as capable as control mice in efficiently eliminating the organism from their contralateral tumor-free foot pad, and from their lymph nodes and livers. The results serve to show, therefore, that conditions within an established tumor are not only antagonistic to the expression of concomitant anti-tumor immunity, but that they are also antagonistic to the expression of T-cell-mediated anti-bacterial immunity. The possibility was discussed that the tumor contains factors that act pharmacologically to locally suppress the function of sensitized T cells and activated macrophages.