Mouse Interleukin-1 Receptor Antagonist (IL-1Ra) is an anti-inflammatory cytokine produced by monocytes, macrophages, neutrophils, and other immune and stromal cells in mice (Mus musculus) that functions as a key endogenous regulator of IL-1–mediated inflammation. IL-1Ra competitively binds to the IL-1 receptor type 1 (IL-1R1) without activating downstream signaling, thereby inhibiting the pro-inflammatory actions of IL-1α and IL-1β and limiting activation of NF-κB and MAPK pathways that drive fever, leukocyte recruitment, and tissue inflammation. Murine IL-1Ra plays a central role in maintaining immune homeostasis in experimental models of rheumatoid arthritis (e.g., collagen-induced arthritis), inflammatory bowel disease, sepsis, gout, neuroinflammation, and metabolic disorders. IL-1Ra–deficient mice spontaneously develop inflammatory polyarthritis, highlighting its critical function in controlling IL-1–driven pathology. As both a biomarker and therapeutic analog, mouse IL-1Ra is widely used in preclinical research to study inflammasome regulation, cytokine network balance, and the efficacy of IL-1–targeted therapies, providing foundational insight into inflammatory mechanisms and treatment strategies relevant to human autoimmune and inflammatory diseases.