Epoxyeicosatrienoic acids (EETs), synthesized from arachidonic acid by cytochrome P450 epoxygenases, are converted to dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. EETs exert anti-inflammatory effects. However, the effect of EETs on humoral immunity is poorly understood. The present study is to investigate the potential role of EETs on B cell function and mechanisms. Firstly, we examined the role of EETs on antibody production of splenic B cells from C57BL/6 (male, 8-week-old, n=8) and ApoE-/- mice (male, 8-week-old, n=8) by means of ELISA. Of the 4 EET regioisomers, 8,9-EET (1 uM) significantly decreased the production of IgM (4.2 ± 0.9 vs. 8.0 ± 0.7 ug/ml) and IgG (35.6 ± 1.8 vs. 57.4 ± 1.0 ng/ml). By use of carboxyfluorescein succinimidyl ester (CFSE) dilution assay, we found that the proportion of dividing cells was lower with 8,9-EET (1 uM) (33.2 ± 0.9% vs. 49.3% ± 1.7%). We next examined the effect of 8,9-EET on B-cell survival stimulated with LPS. And the data showed that the percentage of live cells was substantially lower with 8,9-EET (10.2% ± 3.0% vs. 29.9% ± 2.6%). On the other hand, the generation of CD138+ cells was markedly inhibited by 8,9-EET (28.1% ± 4.5% vs. 39.5% ± 0.5%). Furthermore, we found that 8,9-EET decreased sIgG1+ cell production (3.9% ± 0.3% vs. 7.8% ± 0.3%), which indicated that 8,9-EET inhibits the class-switch recombination of B cells. Molecular events involved in class-switch recombination were significantly inhibited by 8,9-EET. Additionally, nuclear p65 NF-kB level was increased within 5 min after LPS stimulation and was inhibited by pretreatment with 8,9-EET. To determine whether 8,9-EET regulates B-cell function in vivo, sEH-/- mice treated with 8,9-EET were intraperitoneally injected with 4-hydroxy-3-nitrophenyl acetyl-OVA (NP-OVA). The spleen size of mice with 8,9-EET infusion (15 ng/h) was smaller at day 6 pos-timmunization. The colocalization of CD19 and PNA from immunofluorescence analysis of spleens revealed fewer germinal centers and smaller structures. In summary, 8,9-EET may inhibit B-cell function in vitro and in vivo, which suggests a new therapeutic strategy for diseases with excess B cell activation.