Alfalfa (Medicago sativa L.) is commonly used as a traditional medicine and functional food. This study investigated the anti-inflammatory potential of alfalfa and the mechanisms involved. The chloroform extract of alfalfa aerial parts inhibited lipopolysaccharide (LPS)-stimulated immune responses more than ether, butanol, or water soluble extracts. Treatment with $1{mu}g/mL$ LPS increased nitrite concentrations to $44.3{mu}M$ in RAW267.4 macrophages, but it was reduced to $10.6{mu}M$ by adding $100{mu}g/mL$ chloroform extract. LPS treatment also increased the concentrations of tumor necrosis factor-${alpha}$, interleukin (IL)-6, and IL-$1{eta}$ to 41.3, 11.6, and $0.78{mu}g/mL$ in culture supernatants of the cells, but these cytokine levels decreased to 12.5, 3.1, and $0.19{mu}g/mL$, respectively, by pretreating with $100{mu}g/mL$ of the extract. ICR mice injected with LPS (30 mg/kg body weight) alone showed a 0% survival rate after 48 h of the injection, but 48-h survival of the mice increased to 60% after oral administration of the extract. Subfractions of the chloroform extract markedly suppressed LPS-mediated activation of the extracellular signal-regulated kinase and nuclear factor kappa-B. Cinnamic acid derivatives and fatty acids were found to be active constituents of the extract. This research demonstrated that alfalfa aerial parts exert antiinflammatory activity and may be useful as a functional food for the prevention of inflammatory disorders.