The stems of Bougainvillea spectabillis Wild (Nyctaginaceae) have been used in folk medicine against hepatitis. Spinasterol, 22, 23-dihydrospinasterol and caffeic acid were isolated from the plant stems and characterized. Caffeic acid has not been previously isolated from this plant but spinasterol has been isolated from the leaves. Caffeic acid was found to be the active principle exhibiting strong inhibition of xanthine oxidase in this study (IC50 = 39.21 microM). In order to study the structure-activity relationship of the phenolics as regards xanthine oxidase inhibition, twelve naturally occurring phenolics (esculetin, scopoletin, scoparone, barbaloin, berberine chloride, sinomenine, osthole, paeonol, honokiol, magnolol, methyleugenol and 6-gingerol) were tested for their inhibitory effects on xanthine oxidase. The results showed that esculetin displayed the strongest activity (IC50 = 28.4 microM), and induced competitive inhibition of the enzyme with respect to the substrate xanthine. The apparent inhibition constant (Ki) of esculetin was 2.369 x 10(-6) M. Since xanthine oxidase serum levels are increased in hepatic and brain tumors, caffeic acid and esculetin should be tested as anti-hepatitis or/and anticancer agents.