Blood of 16 patients with essential thrombocythemia (ET), 9 patients with reactive thrombocytosis (RT) and 13 healthy persons was used for platelet aggregation studies. When the aggregation was induced with adenosine diphosphate (0.01 microM), collagen (0.1 micrograms/ml) or platelet activating factor (PAF 0.5 microM) the plasma of the patients with ET showed significantly decreased aggregation (35%-44% of the value for the control groups). Independent of inhibitors of platelet aggregation, thrombin (0.05 U/ml) caused similar aggregation in healthy controls and patients with ET; patients with RT showed an increase aggregation. Adrenalin-induced aggregation discriminated best between patients with ET and controls. Adrenalin in concentrations ranging from 0.01 micrograms/ml to 100 micrograms/ml caused comparable dose-related amounts of aggregation in healthy controls and patients with RT. Over the whole concentration range, patients with ET showed significantly decreased aggregation (28%-34% of the value for the control groups). This difference proved to be independent of the influence of inhibitors of platelet aggregation. Though concentrations of alpha1-acid glycoprotein never reached inhibitory levels in the plasma of patients with ET (n = 12) they were significantly higher compared with those in normal plasma (n = 12). Fibrinogen concentrations in plasma of ET-patients (n = 12) were in the normal range. Cellular adenosine 3'-5'-cyclic monophosphate concentrations in ET (n = 10) are comparable with normal values (n = 5). The significance of the results for diagnosis and better pathophysiological understanding of ET is discussed.