The basal platelet level of reactive oxygen species and their enhancement following stimulation by different agonists were determined in a selected group of patients with essential thrombocythaemia (ET). Activated platelets had lower levels of superoxide anion and higher intracellular concentrations of hydrogen peroxide than controls. Higher levels of lipid peroxidation induced by N-ethylmaleimide were also observed. Measurement of the most important enzymes generating and scavenging these reactive oxygen species revealed increased specific activities of NADPH-cytochrome c reductase and superoxide dismutase and a decrease in platelet catalase activity in patients with ET. Since an abnormal production of oxygen radicals seems to be implicated in various pathological conditions and aging processes, the increased amount of hydrogen peroxide found in platelets of patients could be involved in some of the platelet alterations described in ET.