The leaf, stem and root extracts of Chromolaena odorata were evaluated for their effect on platelet-activating factor (PAF) receptor binding on rabbit platelets using ³H-PAF as a ligand. The leaf extract demonstrated high PAF receptor binding inhibitory activity of 79.2±2.1% at 18.2 μg/ml. A total of eleven flavonoids were subsequently isolated from the active leaf extract and evaluated for their effects on PAF receptor binding. Eight of the flavonoids exhibited >50% inhibition on the binding activity at 18.2 μg/ml. These flavonoids were identified as eriodictyol 7,4′-dimethyl ether, quercetin 7,4′-methyl ether, naringenin 4′-methyl ether, kaempferol 4′-methyl ether, kaempferol 3-O-rutinoside, taxifolin 4′-methyl ether, taxifolin 7-methyl ether and quercetin 4′-methyl ether. Their IC₅₀ values ranged from 19.5 to 62.1 μM.