Objective. To develop a highly sensitive method for the determination of platelet-rich plasma serotonin concentration for every-day clinical practice. Material and methods. The method was developed for determination of serotonin in plateled-rich plasma. Serotonin was determined using high performance liquid chromatography with fluorescent detection and hematologic analyzer. Results and conclusion. In healthy people, concentrations of platelet serotonin were similar to those reported in literature: from 2.19 to 4.87 nmol/109 platelets. In patients with depression (n=30), platelet serotonin concentrations were lower (from 0.11 to 2.89 nmol/109 platelets) and correlated with the severity of depression. The method may be used in clinical purposes.