Hydrogen peroxide, at concentrations in the micromolar range, can influence platelet function. Although H2O2 causes no detectable platelet aggregation at these concentrations, it can aggregate platelets which have been previously exposed to any of several aggregating agents. H2O2 can enhance bath aggregation and the subsequent disaggregation of platelets, as initiated by ADP. Preexposure of platelets to H2O2 blunts the subsequent response to ADP. Washed platelets are susceptible to H2O2. Catalase-treated H2O2 has no effect on platelet function. H2O2 does not appear to act by altering either the aggregating agent or plasma thrombin generation. These data suggest that minute amounts of H2O2, which could theoretically be generated in vivo at sites of platelet plug formation, could play a regulatory role in the dynamics of growth and/or dissipation of the hemostatic plug. The blood granulocyte could participate in hemostasis and thrombosis via H2O2 generation.