Previously, we reported that the isoflavones tectorigenin and tectoridin, a glycosylated tectorigenin, isolated from the rhizomes of Belamcanda chinensis have an activity to inhibit prostaglandin (PG) E2 production in 12-O-tetradecanoylphorbol 13-acetate (TPA)-stimulated rat peritoneal macrophages. The inhibitory effect of tectorigenin is more potent than that of tectoridin. In this study, we investigated the structure-activity relationship of various isoflavones in the inhibition of PGE2 production in TPA-stimulated rat peritoneal macrophages. The isoflavones examined were isolated from the rhizomes of B. chinensis, and the flowers and the rhizomes of Pueraria thunbergiana. The order of potency to inhibit PGE2 production was as follows; irisolidone, tectorigenin > genistein > tectoridin (tectorigenin 7-glucoside), glycitein > daidzein. Kakkalide (irisolidone 7-xylosylglucoside), glycitin (glycitein 7-glucoside), daidzin (daidzein 7-glucoside), puerarin (daizein 8-glucoside), and genistin (genistein 7-glucoside) showed no significant inhibition. These findings indicated that 6-methoxylation and 5-hydroxylation increase the potency to inhibit PGE2 production and 7-O-glycosylation decreases the inhibitory activity.