The plasma concentrations of platelet factor 4 were measured in ten patients with septicaemia. In the acute phase the platelet factor 4 values were elevated (27.1 +/- 20.1 ng/ml; mean +/- SD; normal values 4.7 +/- 1.2 ng/ml), and declined significantly after treatment with antibiotics and surgical therapy, respectively (10.6 +/- 7.3 ng/ml; p less than 0.005). The determination of prothrombin time, fibrinopeptide A, AT III and platelet counts demonstrated a coagulation disorder in the acute phase of septicaemia in all patients; a thrombocytopenia (less than 100,000/mm3) was observed in three cases. We suggest that the measurement of platelet factor 4 concentration could be useful as an in vivo indicator and as a follow-up parameter of increased platelet activation.