SUMMARY.Platelet aggregation responses to adenosine diphosphate, adrenaline, collagen, arachidonic acid and ristocetin were measured in healthy subjects, predominantly blood donors residing in Riyadh, the capital city of Saudi Arabia. They were divided according to ethnic origin into Saudi Arabs n = 517, Westerners (Europeans and Americans) n = 93, South East Asians (Koreans and Filipinos) n = 154, and West Africans n = 77. Significant differences in the aggregation responses were found between the four ethnic groups. Saudi Arabs and Westerners showed better aggregation responses to ADP than Asians and Africans. Aggregability in response to collagen was greater in Saudis and Africans than in Westerners and Asians. There was remarkable inhibition of adrenaline induced-aggregation in Asians while other populations produced comparable results. Inhibited responses to arachidonic acid were most prevalent among Westerners and to a lesser extent in Asians than in Saudis and Africans. Ristocetin-induced aggregation was significantly inhibited in Africans and less so in Asians and Arabs and most pronounced in Westerners. These variations in platelet aggregability, which could not be related to blood group distribution, smoking habits, income, physical parameters of height and weight may be due to genetic and dietary factors. These ethnic differences should be taken into account when assessing aggregation responses in patients.