Human platelets were incubated alone or in whole blood with specific agonists such as thrombin or collagen, and 12-hydroxy-heptadecatrienoic (HHT) and 12-hydroxy-eicosatetraenoic (12-HETE) acids were measured by HPLC as indices of platelet cyclooxygenase and lipoxygenase activities, respectively. We found that both arachidonic acid metabolites are significantly formed at concentrations of thrombin insufficient to provoke platelet aggregation. The ratio HHT/12-HETE varied with increasing concentrations of thrombin, with an increase in the absence and a decrease in the presence of albumin in the incubation. When platelets were stimulated in whole blood, this ratio favoured HHT and the addition of albumin to isolated platelets had the same effect. The formation of oxygenated products of 12-HETE by leukocyte LTB w-hydroxylase and 5-lipoxygenase in unstimulated and stimulated leukocytes, respectively, was also investigated. We failed to detect any significant amounts of these products in whole blood incubated with relatively high concentrations of collagen in the presence or absence of the chemotactic peptide FMLP. We conclude that, although 12-HETE is a good substrate for leukocyte oxygenases when incubated at high concentration with the cells alone, its oxygenation is unlikely to occur in whole blood, making 12-HETE and/or HHT potential markers of platelet activation in vivo, provided they are not substantially degraded during passage of the blood through various organs.