The control of the infections caused by Listeria monocytogenes, considered an example of an intracellular parasite, is thought to involve co-operation between antigen-specific T cells and activated macrophages. Here we investigated the participation of polymorphonuclear leucocytes in the mechanisms of resistance during the immune phase of the antimicrobial response to L. monocytogenes infection. We found that BALB/c mice were unable to express T-cell-mediated (acquired) immunity to this pathogen in the absence of granulocytes. We propose that neutrophils should be included in the concept of cell-mediated immunity and that their antimicrobial role is not exclusively expressed during the early phases of a primary infection.