Hydrogen peroxide (H2O2) triggers activation of platelets 'primed' by low concentrations of arachidonic acid (<20μM) or collagen (<0.2μg/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 A2or release serotonin) by H2O2alone or by the low concentrations of arachidonic acid or collagen.The H2O2concentration used (0.15-7.5μM) induces aggregation, TA2production and dense granule content release (monitored by radiolabeled serotonin) by 'primed' platelets. Using arachidonic acid as the 'priming' stimulus, Kappof 687 nM and 560 nM are calculated for platelet aggregation and TA2formation respectively. With collagen as the 'priming' stimulus, Kappof 841 nM and 946 nM are obtained for platelet aggregation and TA2production, respectively.The effect of H2O2is dependent on arachidonic acid metabolism because aspirin prevents H2O2-mediated platelet activation. Furthermore this activation seems to be dependent on arachidonic acid mobilization from platelet phospholipids by phospholipase A2since mepacrine is able to block H2O2-mediated platelet activation.