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Abstract:Of 27 Korean seaweed species screened for potential anti-H. pylori activity, seven (25.9%) showed strong inhibitory activity based on the agar diffusion method. The strongest activity was observed for ethanol extracts from Ishige okamurae. At 1 mg/disk, the inhibition zone of I. okamurae extract was 9.0 mm, and the minimum inhibitory concentration was 12 μg/ml based on the broth microdilution assay. Based on the free urease assay system, the 80% methanol extracts from I. okamurae had 75.4% inhibition at 0.1 mg/ml. To identify the primary active compounds, I. okamurae powders were successively fractionated according to polarity into fi ve classes of constituents including saccharides, lipids, phenolics, alkaloids, and nitrogen compounds. The I. okamurae phenolic compounds had signifi cant antimicrobial activity (12 μg/ml minimum inhibitory concentration), while the nitrogen compound extract signifi cantly inhibited H. pylori urease activity (80.84% at 1 mg/ml). We evaluated the I. okamurae ethanol and 80% methanol extract for acute toxicity in BALB/c mice. Over the 2-week observation period, no death occurred in any mouse administered a dose of 5 g/kg body weight. These results suggest that I. okamurae extract can be used to develop therapeutic agents for chronic gastritis and peptic ulceration.