In order to clarify the modern anti-allergy activity of crude extract of Ecklonia cava, which is an edible marine brown alga used as food ingredient and folk drug against allergy disease in Korea due to the abundant content of phlorotannin derivatives, the continuous chemical investigation led to the isolation of the two main bioactive phlorotannin derivatives 6,6'-bieckol (3) and 1-(3',5'-dihydroxyphenoxy)-7-(2 '',4 '',6-trihydroxyphenoxy)-2,4,9-trihydroxydibenzo-1,4-dioxin (4) for the first time from this genus, together with phloroglucinol (1) and dieckol (2). Their structures were confirmedly elucidated by the comprehensive analyses of MS and NMR spectroscopic data. Meanwhile, these derivatives were assessed by histamine release assay on human basophilic leukemia (KU812) and rat basophilic leukemia (RBL-2H3) cultured cell lines, respectively. Furthermore, flow cytometric analysis indicated that the potential anti-allergic mechanism is the suppression of binding activity between IgE and Fc epsilon RL Finally, the relationships of structure-activity were also suggested. This research indicated that these main derivatives can be used as potential candidates for foodstuff, cosmetic and pharmaceutical industries. (C) 2008 Published by Elsevier Ltd.