To evaluate the antiallergic effect of fermented Ixeris sonchifolia (IS, family Compositae), we prepared IS Kimchi, isolated Lactic acid bacteria (LAB) from it, fermented IS with these LAB, and investigated their antiallergic effects. IS Kimchi more potently inhibited the passive cutaneous anaphylaxis (PCA) reaction induced by an IgE-antigen complex as well as the scratching behavior induced by compound 48/80 or histamine than IS. When IS was fermented with LAB isolated from IS Kimchi, its antiallergic effects was also increased. Of LAB used for fermentation, Lactobacillus brevis more potently increased the antiallergic effects. Its main constituents, chlorogenic acid and luteolin potently inhibited PCA reaction induced by IgE-antigen complex as well as pruritus induced by compound 48/80 or histamine. These constituents inhibited the expression of proinflammatory and allergic cytokines, TNF-alpha and IL-4, and transcription factor, NF-kappaB, activation induced by IgE-antigen complex in RBL-2H3 cells, as well as the degranulation of RBL-2H3 cells induced by an IgE-antigen complex. Luteolin more potently inhibited these allergic reactions than chlorogenic acid. These findings suggest that antiallergic effect of IS can be increased by LAB fermentation and fermented IS might improve allergic reactions, such as pruritus, anaphylaxis, and inflammation.