Hydrogen peroxide (H(2)O(2)) triggers activation of platelets 'primed' by low concentrations of arachidonic acid (< 20 μM) or collagen (< 0.2 μg/ml), but has no effect on platelets exposed to low concentrations of thrombin, ADP or A23187. Platelets are not affected (they do not aggregate or produce thromboxane A(2) or release serotonin) by H(2)O(2) alone or by the low concentrations of arachidonic acid or collagen. The H(2)O(2) concentration used (0.15-7.5 μM) induces aggregation, TA(2) production and dense granule content release (monitored by radiolabeled serotonin) by 'primed' platelets. Using arachidonic acid as the 'priming' stimulus, K(app) of 687 nM and 560 nM are calculated for platelet aggregation and TA(2) formation respectively. With collagen as the 'priming' stimulus, K(app) of 841 nM and 946 nM are obtained for platelet aggregation and TA(2) production, respectively. The effect of H(2)O(2) is dependent on arachidonic acid metabolism because aspirin prevents H(2)O(2)-mediated platelet activation. Furthermore this activation seems to be dependent on arachidonic acid mobilization from platelet phospholipids by phospholipase A(2) since mepacrine is able to block H(2)O(2)-mediated platelet activation.