Low-density lipoproteins (LDL) have been shown to cause aggregation of human blood platelets at concentrations above 2 g of protein/l. The secretion of the contents of platelet dense granules was detected, but not that of the lysosomes. LDL gave rise to a mobilization of [3H]arachidonic acid from phospholipids and the appearance of products of the cyclo-oxygenase pathway after only 10 s. LDL-promoted aggregation was inhibited by both aspirin and indomethacin. There was an increase in 3H-labelled diacylglycerols and the phosphorylation of 47 kDa proteins. LDL therefore shares at least some of the mechanisms of stimulus/response coupling with those of other agonists.