The anti-inflammatory activities of kakkalide, a major constituent of the flower of Pueraria thunbergiana, and irisolidone, a metabolite of kakkalide produced by intestinal microflora, against carrageenan-induced inflammation in air pouches on the backs of mice and in lipopolysaccharide (LPS)-stimulated peritoneal macrophages were investigated. Kakkalide and irisolidone down-regulated the gene expression of cytokines [tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-1β)] and cyclooxygenase-2 (COX-2) and the production of pro-inflammatory cytokines, TNF-α and IL-1β, and inflammatory mediators, NO and prostaglandin E(2) (PGE(2)), in LPS-stimulated peritoneal macrophages. These agents also inhibited the phosphorylation of IκB-α and the nuclear translocation of nuclear factor-kappa B (NF-κB). Orally administered kakkalide and irisolidone significantly reduced carrageenan-induced inflammatory markers, leukocyte number, and protein amount in the exudates of the air pouch. These constituents also inhibited PGE(2) production and COX-2 inducible nitric oxide synthase, IL-1β, and TNF-α expression. These agents also inhibited NF-κB activation. The anti-inflammatory effects of irisolidone were more potent than those of kakkalide. Based on these findings, kakkalide and irisolidone may inhibit inflammatory reactions via NF-κB pathway, and irisolidone, a metabolite of kakkalide, may more potently inhibit these inflammatory reactions.