Our previous study showed that Leukotriene B4 can directly stimulate osteoclast differentiation independent of RANKL. In order to determine whether Leukotriene B4 could indirectly stimulate human osteoclast differentiation through increasing RANKL expression of rheumatoid arthritis fibroblast-like synoviocytes, we utilize the coculture model of rheumatoid arthritis fibroblast-like synoviocytes and monocyte, which were stimulated in the presence of 2.5 ng/ml M-CSF in the control group, 2.5 ng/ml M-CSF+10(-8)M LTB4 in the experimental group a, and 2.5 ng/ml M-CSF+10(-8)M LTB4+100 ng/ml OPG in the experimental group b. After culture for 3 weeks, the number of multinucleated TRAP staining positive osteoclast-like cells stained with TRAP was counted to evaluate the differentiation effect in each group. There was almost no osteoclast-like cell in the control group and the experimental group b. There were many osteoclast-like cells in the experimental group a. These results indicated that Leukotriene B4 is capable of inducing osteoclast differentiation by a RANKL-dependent mechanism.