The elements Ag, Au, Cd, Co, Cr, Cs, Mo, Rb, Sb, and Se were determined in platelets from seven normal donors. The results, in ng/g wet weight, for plasma-free platelets follow: "Pure" platelets: Ag = 29 +/- (18), au = 0.22 +/- (0.22), Cd = 6.2 +/- 3.4, Cs = 54.8 +/- 19.2, Cr = 6.1 +/- 2.5, Co = 7.5 +/- (5.0), Mo = 3.4 +/- 1.3, Rb = 10400 +/- 3000, Sb = 18 +/- (26), and Se = 782 +/- 127. "Impure" platelets: Au = 0.23 +/- (0.28), Cd = 6.4 +/- 2.6, Cs = 35.2 +/- 13.8, Cr = 8.2 +/- 2.9, Co = 2.9 +/- (3.0), Mo = 3.2 +/- 0.8, Rb = 8700 +/- 1700, Sb = 13.2 +/- (8.7), and Se = 679 +/- 57. To our knowledge, none of these 10 trace elements has been determined in platelets before. The selenium concentration in platelets exceeds that in other tissues (e.g., liver). We suggest that glutathione peroxidase or other unknown selenoenzymes are particularly important in platelet metabolism. Platelets are crucial for triggering thrombosis, and so may be involved as links between selenium deficiency and the concomitant increased death rate from cardiovascular disease.