We investigated the role of leukotriene B 4 (LTB 4 ) in murine arthritis models using a leukotriene A 4 (LTA 4 ) hydrolase inhibitor, SA6541. SA6541 inhibited the severity of collagen-induced arthritis and muramyl dipeptide (MDP)-induced hyperproliferation of synovial cells in vivo. SA6541 also inhibited LTA 4 -induced hyperproliferation of synovial stromal cells in vitro. These results suggest that LTB 4 may play an important role in arthritis models.