Most of the blood serotonin is bound to platelets. However, some authors described plasma "free" serotonin; which is normally very low, and might be increased during some diseases. Usual spectrofluorimetric methods and paired by statistical evaluation "extra-platelet" serotonin (not bound to platelets) in the blood of healthy volunteers and patients with myeloproliferative disorders was studied. In these conditions, we cannot be sure of the existence of "extra-platelets" serotonin in the blood of controls and some patients; in others it was present. "Extra-platelet" serotonin is always present in the blood of untreated patients with primary thrombocytosis.