Triggering receptor expressed on myeloid cells (TREM) 1 is an activating receptor expressed on myeloid cells whose ligand(s) remain elusive. TREM-1 stimulation activates neutrophils and monocytes and induces the secretion of pro-inflammatory molecules, which amplifies the Toll-like receptor-initiated responses to invading pathogens. In addition, TREM-1 mediates the septic shock pathway, and thus represents a potential therapeutic target. We report the crystal structure of the mouse TREM-1 extracellular domain at 1.76A resolution. The mouse extracellular domain is monomeric, consistent with our previous human TREM-1 structure, and strongly supports the contention that the globular TREM-1 head is a monomer contrary to proposals of a symmetric dimer.