A ninhydrin positive compound (L2) from commercially available unfermented dry green tea (Thea sinesis) leaves is found to be a potent inhibitor of thrombin-stimulated thromboxane formation in rabbit whole blood. Its potency is compared with caffeine, a member of the methylxanthines family. Both caffeine and L2 inhibit thromboxane formation in whole blood in a dose dependent fashion. L2 inhibition when calculated as I50 by a dose response curve is found to be more than 40 fold stronger than caffeine as an inhibitor of thromboxane formation. A concentration of L2 as low as 50 microM, suppresses thromboxane formation (by 84%) whereas a concentration of 5000 microM is necessary to achieve the same inhibition with caffeine. The potent inhibitory effect of L2 on the TXB2 production maybe of benefit in the treatment of vascular disease.