An open study to investigate the PAF-acether antagonist activity of Tanakan in healthy male volunteers examined the effect of a single 15-ml oral dose on ex vivo platelet aggregation induced by adrenaline, adenosine diphosphate (ADP), collagen or PAF-acether. Aggregometry was performed on platelet-rich plasma samples from blood taken 1 h before dosing with Tanakan and 2, 4 and 8 h after intake of Tanakan. Following dosing with Tanakan there was a reduction in platelet aggregation at all doses of PAF, with 1 microM ADP and adrenaline, the most significant decreases occurred with 75 nM PAF-acether 4 h after intake (p less than 0.05) and 300 nM PAF-acether 4 h (p less than 0.01) and 8 after intake (p less than 0.05). There were no concomitant changes in coagulation, skin bleeding time, haematological and biochemical laboratory tests, blood pressure or pulse. The results provide a possible explanation for the clinical efficacy of Tanakan in the treatment of peripheral vascular disease; they also confirm that a single oral dose of 15 ml Tanakan is well tolerated.