As part of a series of biological examinations of various tannins and related compounds, the present paper reports the effects of caffeetannins and related compounds isolated from medicinal plants on arachidonate metabolism in human peripheral polymorphonuclear leukocytes (PMN-L). The formation of leukotriene B4 (LTB4) induced by calcium ionophore A 23187 (A 23187) in human PMN-L was inhibited by 3,5-, 4,5-, and 3,4-di-O-caffeoylquinic acid, caffeoylmalic acid, caffeoyltartric acid, rosmarinic acid, and caffeic acid. Rosmarinic acid strongly inhibited the formation of 5-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE) and LTB4 (5-lipoxygenase products) at concentrations of 10(-5)-10(-3) M. On the other hand, the formation of prostaglandin E2 (PGE2) was enhanced in a concentration-dependent fashion by caffeic acid, caffeoylmalic acid, caffeoyltartaric acid, and 3,4-di-O-caffeoylquinic acid. On the basis of experimental results, it seems likely that caffeoyl derivatives could be developed as therapeutic drugs for treatment of allergic inflammation such as asthma.