Neutrophil granulocytes play an important role in innate host defence against microbial invasions and they are also the key effector cells in mediating host tissue damage. These functions often rely on the production of reactive oxygen species (ROS) from the membrane-bound NADPH-oxidase system. The magnitude of ROS production varies depending on the state of the cells, i.e. resting or primed. Many priming agents as well as potent NADPH-oxidase activators have been identified and characterized for human neutrophils. The cytokine tumour necrosis factor (TNF)-alpha is one prominent example of a priming agent and the synthetic hexapeptide WKYMVm is an agonist that triggers an activation of the NADPH-oxidase of human neutrophils through two members of the formyl peptide family of receptors, formyl peptide receptor (FPR) and FPR-like 1 (FPRL1). This peptide also activates murine neutrophils but the precise receptor involved has not been previously characterized. We show in this study that WKYMVm activates stably transfected HL60 cells expressing murine formyl peptide receptor-related sequence 2 (Fpr-rs2) and that activation of murine neutrophils with WKYMVm is blocked by an FPRL1-specific antagonist. WKYMVm is thus an agonist for Fpr-rs2 and we suggest that this receptor is in fact the mouse orthologue of FPRL1. In addition, we show that the WKYMVm response in murine neutrophils can be primed by TNF-alpha and this priming process involves mobilization of subcellular granules. The results obtained using neutrophils derived from TNF receptor type I (TNFRI)-deficient animals suggest that TNF-alpha exerts its priming effect via the TNFRI.