This chapter focuses on deglucuronidation by beta-glucuronidase in inflammation. We investigated whether glucuronides were converted to free parent compounds by beta-glucuronidase released from human-stimulated neutrophils in inflammation. Beta-glucuronidase activity was assayed using 4-methylumbelliferyl-glucuronide and methanol extracts of rat plasma containing luteolin monoglucuronide as a substrate. The released 4-methylumbelliferone, a fluorescent molecule, was quantitated on a microplate fluorometer. Deglucuronidation of luteolin monoglucuronide was examined by high-performance liquid chromatography (HPLC) analysis. The beta-glucuronidase activity in mouse plasma after iv injection of lipopolysaccharide (LPS) increased with time, as did the levels of inflammation marker, tumor necrosis factor-alpha, and soluble intercellular adhesion molecule-1. Four kinds of human cell (neutrophils, human umbilical vein endothelial cells, IMR-90, and Caco-2) possess beta-glucuronidase activity. Among these, Caco-2 cells showed the highest level of beta-glucuronidase activity. Supernatants obtained from neutrophils stimulated with cytochalasin B and ionomycin showed higher levels of beta-glucuronidase activity than those of nonstimulated neutrophils. HPLC analyses also showed that supernatants obtained from stimulated neutrophils hydrolyzed luteolin monoglucuronide to free luteolin. As reported previously (Shimoi et al., 1998), two main peaks (free luteolin and luteolin monoglucuronide) were observed in plasma of rats administered with luteolin. In LPS-treated rats, the peak of luteolin monoglucuronide decreased to about half and the ratio of luteolin to luteolin monoglucuronide increased. These results suggest that beta-glucuronidase released from neutrophils or certain injured cells may hydrolyze glucuronide conjugates to free aglycones at the site of inflammation.