Acquired resistance to the intracellular bacterial parasite, Listeria monocytogenes can be transferred to normal recipients by thoracic duct lymphocytes or peritoneal exudate cells obtained from rats infected with this organism; The appearance of protective cells in thoracic duct lymph coincides with the development in the donors of delayed-type hypersensitivity to Listeria antigens and accumulation in induced peritoneal exudates of cells which are responsive to these antigens in the migration inhibitory factor (MIF) assay. The cells in exudates that confer protection, and those that release MIF, arise at sites remote from their final destination. From their point of origin in the caudal lymph nodes of infected rats, cells with these properties are delivered to the thoracic duct and hence to the blood from where they are drawn into the peritoneal cavity in response to inflammation. The parallel observed in the appearance, increase and subsequent decline of protective lymphocytes and MIF-producing cells in exudates suggest that the two activities are mediated by a single line of T cells. However this may be, the development and deployment of the cells concerned encourages the belief that MIF has a meaningful role in the expression of cellular resistance to infection.